Tail Light / License Plate Light Problem / Fix
I own a 1999 Grand Cherokee, and I get a lot of my troubleshooting information from sites like this one. Most recently, I could not find any accurate or satisfactory answers to the tail light / license plate light / parkng light fuse and bulb blowing problem that I, like many others, had experienced. A few days ago, I had a day off to just experiment, and here is what I learned. My problems are now fixed, by the way.
Problem: I kept replacing tail light and license plate bulbs and fuses (15 amp number 6 fuse on fuse block). Sometimes, the problem would appear to have fixed itself for days, but then, without warning, I would suddenly have no tail lights or license plate lights. Sometimes, the bulbs were blown, but most often, it was the 15 amp parking light fuse that I had to replace. On bad days, I might have to do this twice or three times. I was advised by a dealership to replace the tail light units and even the wiring harness, but in the end, the fix cost me almost nothing, and I did everything in my driveway.
1) I removed headlight/parking light units, tail light units, and two license plate light units and tested them, but all units were fine.
2) I bought a 25-count box of 15 amp fuses for the purpose of testing, then began turning on the parking lights with only certain light bulbs inserted. First front parking, then tail lights, then license lights, then combining them in that order. No matter which light bulbs were present, they blew the fuse almost immediately each time, but I noticed that if I just backed off and never touched the car at all, they tended to work fine. Any movement of the car would blow the fuse. (E.g. opening or closing a door or sitting down in a seat--even rocking the vehicle.)
3) I removed enough of the lighting units and interior panels to make the wiring harnesses accessible. Then, I repeated my testing, wiggling the harnesses one at a time, hoping to find a short somewhere. Nothing changed, but quite without intention, I moved the open liftgate downwards to get a better view of the two license plate bulbs. The fuse promptly blew.
4) I replaced the fuse, got the lights all working, and moved the liftgate ever so gently. The fuse blew.
5) I removed the interior panel from the liftgate, along with the well-glued rubber weather panel. This exposed the wiring harness within the liftgate itself.
6) I repeated step 4, this time moving only the wiring harness inside the liftgate. The fuse blew.
7) I detached the clips holding the wiring harness to the liftgate body and ran my fingers along the wiring harness, and sure enough, directly even with the electric lock actuator housing, I discovered a bare wire with its insulation worn or cut away. The actuator housing is sharp-edged, and the wiring harness was installed (from the factory) pressing tightly against one sharp edge. After over a decade of rubbing, the insulation had been penetrated, and bare wire was against the metal (grounded) housing. When the liftgate was moved, even ever so gently, or when the vehicle hit a bump or turned a corner or was rocked, the wiring harness would short out against the metal liftgate lock actuator housing and blow the fuse.
8) I re-routed the liftgate wiring harness slightly to prevent a reoccurence, then wrapped the wire well with electrical tape to keep it from touching anything else ever again.
9) At this point, I replaced the fuse and tried everything I could think of to make it blow again, but it was solid as a rock.
10) I put everything back together, using auto weatherstrip adhesive to stick the weather panel back to the liftgate interior prior to replacing the interior panel.
Since fixing the problem, I have not experienced any further similar lighting issues. I'm guessing this may be one of several poorly routed wiring harness spots that end up shorting out lighting and other electrical components. I'd be interested to know if this solves anyone else's similar problems. Keep in mind that the harness may be rubbing against some other frame or body component elsewhere, not just in the liftgate, but that's a good starting place for investigation.
Good luck, and have fun with all your Grand Cherokee troubleshooting endeavors!