A dead XJ rear defroster is almost never the glass — it's a broken grid line, a popped bus bar tab, or a liftgate wire that fatigued at the hinge, and all three cost under $30 to fix with a multimeter and conductive epoxy.
The grid is two vertical bus bars with horizontal heating elements fired onto the inside face of the glass. Power enters the driver's-side bus bar through the defogger relay, current runs across every horizontal line, and the passenger-side bus bar carries it to a body ground. Break one line and you get a clear-glass stripe across the window while the rest works. Lose a tab, a wire, or the relay and the whole window stays fogged. Distinguishing those two failures takes about ten minutes, and doing it before you buy anything keeps you from paying a glass shop for a window that has nothing wrong with it.
One thing to rule out first: the XJ defogger is on a timer. The bright red timer relay runs the grid for roughly 10 minutes on the first press, then about 5 minutes if you press it again during the same ignition cycle. A defroster that shuts itself off is behaving exactly as designed.
1984-1996. Per the factory manual, the grid feed is protected by a 25 A fuse (No. 18) and the relay's power comes through a 20 A fuse (No. 8) in the interior fuse panel. The red timer relay lives behind the instrument panel to the right of the steering column, behind the lower trim.
1997-2001. The defogger moves into the junction block behind the lid under the glove compartment. Fuse 12 (10 A) feeds the defogger relay coil and shares with the combination flasher; fuse 6 (5 A) feeds the switch. The relay itself is position R4 in that junction block. The high-current grid feed comes from the under-hood Power Distribution Center — a 30 A fuse in cavity 10 on 1997-1998 rigs (shared with the starter relay) and cavity 11 on 1998-2001.
Fuse-panel legends vary between model years and printings, so read the label on your own panel before pulling anything. If the grid feed fuse is intact but there's no battery voltage on its supply side, the failure is upstream in the PDC.
Work in this order — it goes fastest to slowest.
Confirm the switch indicator lights. No light points at the switch, fuse 6, or the relay's 5 V signal circuit.
Turn the defogger on, engine running, and probe the driver's-side bus bar to a known-good chassis ground. Battery voltage there means power is arriving and the problem is downstream.
Probe the passenger-side bus bar to ground with the system off, measuring resistance. Anything but near-zero ohms means the ground-side wire or its body ground is open. On a rust-belt XJ this is the most common whole-window failure, and it usually lives at the ground eyelet behind the passenger-side quarter trim.
Inspect both pigtails where they attach to the bus bars. XJ tabs pull free when somebody yanks the wire moving cargo, and the break is often hidden under the headliner edge.
Flex-test the liftgate harness at the rubber boot between the body and the hatch. Two decades of hatch cycles work-harden the copper inside intact-looking insulation. Wiggle the loom with the defogger on and watch for the voltage reading to drop out.
Swap in a known-good red timer relay before condemning anything expensive.
The clear stripe tells you which line is broken, not where. The factory procedure finds the exact spot:
Engine running, defogger on. Put the meter's negative lead on the ground-side bus bar and the positive lead on the feed-side bus bar. You should see 11-13 V.
Move the negative lead to a good chassis ground and leave it there. The reading should hold steady.
Touch the positive lead to the dead grid line at its centerline. About 6 V — half the supply — means that half is intact. 0 V means the break is between your probe and the ground side. 12 V means the break is between the feed side and your probe.
Slide the probe left or right along the line. The point where the reading changes suddenly is the break. Mark it on the outside of the glass with a marker so the marking doesn't interfere with the repair.
Ice scrapers used from the inside are the leading cause, followed by cargo shifting against the glass on the trail and rear seats folded flat with gear sliding into the window. Window tint removal is the other big one — pulling old film off an XJ hatch takes grid lines with it, so tint over a defroster grid is effectively permanent.
If more than three or four lines are broken, individual repairs stop making sense. A copper grid tape kit bonded over the glass restores function at the cost of appearance, and a salvage-yard hatch glass with an intact grid usually runs less than a professional repair.
Steps
Clear the cargo area and work from inside with good light. Do not perform this repair below about 60°F — conductive epoxy cures poorly in the cold, and a rear window that has been out in the weather needs to warm up first.
Rub the marked area lightly with #0000 steel wool or a nylon pad to remove oxidation from the broken ends. Light pressure only — the grid line is a fired-on silver ceramic film a few thousandths thick, and aggressive scrubbing removes more line than it cleans.
Wipe with isopropyl alcohol, let it flash off, then wipe again. Any wax, glass cleaner residue, or Armor All on that glass will keep the repair from bonding.
Lay masking tape above and below the grid line, leaving a channel the exact width of the original line. If your kit includes a stencil, use it instead — the goal is a repair no wider than the line it replaces, because a wide blob changes the resistance of that circuit.
Mix the conductive epoxy thoroughly. Silver-filled epoxy separates in the tube, and an unmixed batch reads as an open circuit after it cures.
Apply over the break so the new material overlaps at least 1/4 in onto sound grid line on both sides. Overlap is what makes the joint conduct; a butt joint fails.
If you're using conductive paint rather than epoxy, build up 6-8 thin coats, letting each flash off between passes. Paint carries far less current per unit thickness than the original grid, so a single coat runs hot and burns open.
Pull the tape while the material is still wet, straight up and away from the line.
Cure 24 hours at room temperature, or hold a heat gun about 10 in from the glass for 15 minutes. Do not hold the gun closer — thermal shock cracks tempered glass and the repair is not worth a new window.
For a popped bus bar tab, use a tab kit with cleaner, activator, and conductive adhesive rather than a soldering iron. Solder works — 60/40 rosin core, in and out fast — but the heat cracks glass often enough that the adhesive route is the better bet on a 25-year-old window. Clamp or tape the tab under light pressure for the full cure.
Reconnect everything, run the system, and verify with the meter: about 6 V at the centerline of the repaired line. Then feel the glass — the repaired stripe should warm at the same rate as its neighbors.
Mark the repair location in your notes and treat that area as fragile from here on. No scraping, no ice, no glass cleaner scrubbed across it, no cargo leaning on it.
Why it works
Restores a function most XJ owners write off as unfixable, for the price of a tube of epoxy
The diagnosis is transferable — the same voltage-halving method finds a break in any vehicle's grid
No glass removal, no interior teardown beyond trim panels
Fixing the ground-side wire or a popped tab often brings back the entire window in under an hour
Catches a fatigued liftgate harness before it takes out the rear wiper and third brake light too
Trade-offs
The repair is mechanically delicate forever after and will not survive a scraper
Conductive paint repairs carry less current than the original line and can burn open again
Cure time means the vehicle sits overnight unless you use a heat gun
Below about 60°F the material won't cure properly, which is when you most want the defroster
Not viable past a handful of broken lines — at that point it's replacement glass or a copper grid overlay
Soldering a bus bar tab risks cracking tempered glass, and a cracked hatch glass costs far more than the repair saved
Tools required
Digital multimeter (DC volts, 20 V range)
Fine steel wool (#0000) or a nylon abrasive pad
91% isopropyl alcohol and lint-free rags
Masking tape
Fine-tip permanent marker
Heat gun (optional, to speed the cure)
Trim panel removal tool (liftgate and quarter trim access)
Test light
Parts
Part
Vendor
Est. price
Conductive grid repair kit — silver paint plus grid-line stencil
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.