Getting a working 4-way flat on the back of an XJ comes down to one thing most people get wrong: the Jeep runs separate stop and turn circuits, so you need a converter, and that converter needs a real ground.
Trailers, utility haulers, and a second rig on a tow dolly all need the same four functions from the tow vehicle — running lights, brakes, left turn, right turn. The XJ, like every US-market Chrysler product of the era, uses a five-wire rear lighting layout with separate stop and turn lamps on each side. A trailer 4-way flat combines stop and turn onto one wire per side. Something has to do that math, and that something is a taillight converter.
Two ways to get there. A vehicle-specific T-connector harness has the converter built in and plugs between the factory taillight connectors and the taillight assemblies, no cutting. CURT lists 55354 for 1997-2001 Cherokee (all models) and 56208 for earlier years — confirm your year against the CURT application guide before buying, because the rear harness connectors changed across the XJ's run. The other way is a universal 3-to-2 converter spliced into the rear harness, which is what you'll end up doing on an early XJ, on a Jeep whose rear harness a previous owner has already butchered, or when you want to place the converter where you can reach it.
If a previous owner has installed a hitch and left cut wire ends dangling behind the driver's-side quarter trim, treat that as a rewire, not a repair. Vampire taps and twisted-and-taped splices wick water into 25-year-old copper and will fail on the highway with a trailer behind you.
Vehicle-specific T-connector (55354 / 56208). Fastest and the least invasive — plugs into the OEM taillight connectors and gives you a 4-way flat pigtail with a built-in circuit-protected converter. Note the ratings: the 55354 supports 7.5 A on the tail lamp circuit and 3.0 A on each turn and brake circuit. That is enough for LED trailer lights or a small incandescent utility trailer, and not enough for a big multi-bulb enclosed trailer.
Universal 3-to-2 converter. Splice-in, roughly $20, and it works on any XJ year including 1984-1990 rigs where T-connector fitment is spotty. You are responsible for identifying the circuits correctly and sealing every splice.
Powered converter with a fused battery feed. The converter pulls trailer light current from the battery instead of the XJ's taillight circuit and uses the taillight wires only as signals. This is the answer if your trailer is heavy on incandescent bulbs, if you're already noticing dim rear lights, or if you plan to add a 12 V charge line later. It costs more and takes a fused feed run from the engine bay to the rear.
Do not rely on a stock 4-way flat harness to feed electric trailer brakes. Trailer brakes need a 7-way connector, a brake controller, and a dedicated fused power line — a separate job, and the one that matters most once you're over about 2,000 lb.
A stock XJ without the factory tow package is realistically a 2,000 lb tow vehicle; with the factory tow package — Class III hitch, heavy-duty cooling, appropriate axle ratio — the 4.0 is rated up to 5,000 lb with a 500 lb tongue weight. The XJ's limits are its brakes and its light rear end, not its engine. Once you're pulling enough weight to need trailer brakes, the 4-way flat you're installing today stops being sufficient and you'll want a 7-way and a controller. Plan the wire routing now so you're not doing it twice.
Steps
Disconnect the battery negative terminal. You're working in the taillight circuits and a slipped probe will pop a fuse at best.
Open the rear cargo area and pull the driver's-side rear quarter trim panel. The factory trailer wiring provisions, and the wire bundle you'll be tapping or T-ing into, live behind it near the spare tire well. Pull the passenger-side panel too if you're installing a T-connector, which plugs in on both sides.
Remove the taillight assembly fasteners and pull each lamp housing out far enough to reach its connector. Support the housing rather than letting it hang on the harness.
Identify every circuit before you commit. Reconnect the battery, then use a test light grounded to bare chassis metal to confirm each wire one function at a time: running lamps on, brake pedal pressed, left turn, right turn, reverse. Color codes on an XJ vary by year and get scrambled by prior repairs — a 1998 rear harness commonly shows white, brown, black, green/brown, and green/red, but verify rather than trusting a chart. Write the results down.
Disconnect the battery again before you cut or plug anything.
T-connector route: unplug each factory taillight connector, plug the T-connector's matching halves in between the harness and the housing, and route the converter body and 4-way flat pigtail toward the center of the vehicle behind the trim.
Splice-in route: connect the converter's inputs to the circuits you identified — left stop/turn, right stop/turn, and tail/running — using adhesive-lined heat shrink butt connectors or solder plus adhesive heat shrink. Never use insulation-displacement (vampire) taps here; they sever strands and hold water against the copper.
Ground the converter properly. This is the step that decides whether the install works. Run the converter's white wire to a dedicated self-tapping screw into clean bare unibody metal — scrape the paint and undercoating off first — not to an existing taillight ground splice. Snug the screw to roughly 5-8 ft-lb; that's tight into sheet metal, not gorilla-tight. A converter sharing a marginal ground is the root cause of nearly every "my trailer running lights blink with the turn signal" complaint.
Route the 4-way flat pigtail out through an existing grommet or the factory wiring pass-through in the floor near the spare well. If you must drill, deburr the hole and use a rubber grommet — a raw hole in the floor will cut through insulation over a few thousand miles.
Zip-tie the pigtail along the unibody rail with at least 6 in of clearance from the exhaust and clear of the rear axle's full droop travel. Leave a service loop at the connector so a lifted XJ or a tight turn doesn't stretch it.
Mount the connector on the receiver with a bolt-on connector bracket, or clip it where you can reach it without lying under the bumper. Fit the dust cover — the terminals corrode fast once they've been through one winter.
Reconnect the battery. Test every function individually with a 4-way flat tester or the trailer itself: running lights, left turn, right turn, brakes.
Test the combinations, which is where bad grounds and mis-identified circuits show up. Hold the brake pedal and run the left turn signal, then the right. The trailer's brake side should stay lit while the turn side flashes. Any cross-feed — running lights pulsing with the signal, the wrong side flashing, a dim glow where nothing should be lit — means you're back to step 8.
Check the XJ's own lights afterward. All five rear functions on the Jeep should behave exactly as they did before the install.
Why it works
Roughly an hour of work with hand tools for the T-connector route
No cutting or splicing on a factory harness with the vehicle-specific harness
Built-in circuit-protected converter handles the XJ's separate stop and turn circuits correctly
Opens up utility trailer, tow dolly, and camper use for well under $100
The same rear-harness knowledge carries over to a later 7-way and brake controller install
Trade-offs
Taillight-powered converters are current-limited (7.5 A tail, 3.0 A per turn/brake circuit on the 55354) and will struggle with a bulb-heavy trailer
T-connector fitment for 1984-1996 XJs is inconsistent — many of those rigs need the splice-in route
A marginal ground produces confusing cross-feed symptoms that look like a failed converter
25-year-old rear harness insulation is brittle; expect to repair prior owner damage before the new harness works
Does nothing for trailer brakes, which is the actual limiting factor once you're over about 2,000 lb
Tools required
Phillips and Torx screwdrivers (taillight fasteners)
Trim panel removal tool
Test light or digital multimeter
4-way flat circuit tester
Wire stripper and ratcheting crimper
Heat gun (for adhesive-lined heat shrink)
Metric socket set (8mm, 10mm)
Drill and step bit (only if mounting a ground screw or bracket)
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.