Rear light questions

OK, this may seem like a stupid question to some, but I realized that the rear lights in my 88 aren't working properly. I need to repalce the sockets because they're badly corroded, but I'm trying to figure out what each bulb is supposed to do. I never was behind the Jeep before, so I don't know what comes on with brakes, what flashes for signal, etc. There are 4 bulbs in each light, and I don't know what each one is for. Figuring that out will help me trace the wiring to find where my issues are. Also, I have a 4-pin trailer plug. I believe it is factory because the tow hitch appears to be and there is a tranny cooler. I was under the impression that we had a 5-wire system with separate brake and turn signal bulbs, so that would mean there is an adapter to convert it to run with a 4-wire trailer system. Where would this be located? I think there may be a fault in that because 2 of the pins on my trailer plug are dead, and the other that corresponds to left turn/brake has a constant 7.5v with the signal on and no brakes, which does not make any sense to me.

on my 89, the trailer hitch harness is behind the panel the spare tire is mounted to.

The adapter is where 89xj said. As far as the lights top bulb is tail light & brake light that is an 1157 two filament bulb behind red lens. Next is a single filiment bulb for a second tail light I think is a 67 bulb. Next is a single filiment 1156 bulb for turn signal behind amber lens. Next is a single filiment 1156 bulb for reverse lights behind clear lens. The little bulb in the rubber socket next to the second tail light is a 194 bulb for side marker light. This is the setup for 1987 thru 1990. I seem to remember my buddy's 1992 does not have the second tail light. Hope this helps.
I had put an 1156 bulb in my driver side second tail light in place of the 67 bulb for a european type rear collision avoidance light as I end up in fog quite a bit at night.

That's exactly what I needed, thanks guys. That was kind of how I was thinking it was setup. So basically, the whole red lens is supposed to be lit up when the headlights are on, but only the top gets brighter when you hit the brakes?

RustyBlackXJ said:

That's exactly what I needed, thanks guys. That was kind of how I was thinking it was setup. So basically, the whole red lens is supposed to be lit up when the headlights are on, but only the top gets brighter when you hit the brakes?

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I need to know the answer to this too.....but my xj is an '86