It blew when I went home for lunch; I drove back to work with no taillights. Before leaving the office I put a new fuse in, and it blew before I got out of the parking lot. Stepped on the brake; the rear defogger light came on, dimmed a little bit, dimmed a little more, and then winked out. Five seconds.
Got home and tore into it.
First up, remove the trim panel from the liftgate and unplug one side of the defroster. Same thing: five seconds and pop.
Pulled the left-side taillight assembly and started checking for grounds. Found that all four terminals in the stop lamp were shorted to ground. Realized that it might be feeding through the other side, so I removed it...same thing. Same thing on the right side, too.
Now, it's got three terminals. One for the parking lamp filament, one for the brake lamp filament (there are two filaments in the bulb) and then one for the ground. The bulb has four terminals, and so the socket does, too. I should not have a path to ground from all four of the terminals in the socket. At most, two of them should be ground. And it was the same case on both sides, which really didn't make a hell of a lot of sense given that only the left side parking lamp fuse blows.
Next step, pull the interior trim panel off from behind the spare tire carrier, because that's where the trailer harness box is; and as soon as I got that panel off the stench of roasted electronics hit my nostrils like an 18-pound sledge.
So I pried the trailer harness black box from the double-sided tape holding it in place, and wouldn't you know it, the formerly flat box was bulged in the middle, and it reeked. So I unplugged it from the wiring harness, having to use pliers to get one plug out...and although I didn't blow any fuses when I stepped on the brake, I didn't have brake lights, either.
Pulled the plugs apart again, and--oh, God:
First image is the plug on the Jeep's wiring harness. The second two are the plug from the trailer wiring kit that plug into the first plug. Yeah, and notice how the insulation for the wires was melted. Yeah, no wonder I was blowing fuses; I'm lucky the thing didn't catch fire!
So, scorched plugs, pins melted off, a couple of broken wires--yeah, what a party this is.
I looked at the thing and tried to figure out how to fix it. What I needed was a new male--female--transsexual? Damn it, I don't know what you call it when the pins are inside a housing. I needed a plug that would replace the one in the first picture, though, and my mind went through the possibilities (Amazon? Parts store? I'd have to order it wherever I went; could I just use bullet connectors?) when it hit me: I had a good plug, didn't I? The one from the trailer kit--I mean, I'm sure as hell not putting it back in, and that connector was pristine, so couldn't I just solder some wires together and call it good?
I looked over the two plugs, and realized that this could work, so I started doing it.
...took me a while to get it all done, but I managed to tin the wires, solder them together, and I even remembered to put the heat-shrink tubing on before soldering them. Made sure nothing was touching...and I was able to step on the brake without blowing a fuse. But the left side still was not lighting!
After some bulb swapping and fiddling around I discovered that several of the brake lamps were blown. Well, that doesn't help, does it? So I was going to have to go get some bulbs--that would have to wait until Mrs. Fungus got home, so I started working on finding the heat gun so I could shrink the heat-shrink tubing...and Mrs. Fungus came home then, bearing McDonald's.
After wolfing down dinner I hit the parts store and bought all-new bulbs for the rear end, plus a pair for the high-mount brake lamp. Put the new bulbs in the housings and tested brakes, back-up lamps, and parking lamps. All good, but for the high-mount lamp. I decided I'd dig into that another time, though, because I was fading fast and I needed to get this thing buttoned up before I hit the wall.
I could not find my heat gun. It's not where I remember putting it, and my wider search was fruitless; so I ended up grabbing the propane torch and using it to shrink the heat-shrink tubing. I managed to do so without ruining anything or setting anything on fire, which was a huge plus.
...and in fact all I really did after that was to bolt up the taillamp housings and collect the screws in a container and put everything away. I'm leaving it open in case I need to dive back into it, and also because I was too damned tired to reassemble everything. Let's face it: an 8-hour day at work, plus four more hours of working on the truck afterwards--I was done.
But I stepped on the brake pedal for a full 30 seconds and no fuses blew. All the lights out back are working (but for the high-mount).
Plugged the defroster back in, and the light comes on when I step on the brakes, just like it did before. I think that's a separate issue.
Anyway: so the entire reason that just about all the lights were coming on when I stepped on the brake was due to the trailer harness; the box blew out, and it blew bad such that everything was shorted together. I'm going to need to get another one before I can pull any trailers with the Jeep, but because I retained the original configuration for the plugs in back, it'll be a plug-in deal just like the last one was.
Anyway, that's what it was. I still need to figure out a couple of things back there, like why the liftgate no longer turns on the cabin lights, and that high-mount stop lamp, but it's not an emergency. I've got brake lights, and parking lamps, and that's what counts.
Man, am I beat.