Here is a partial 88 XJ schematic it may be helpful, it is likely close to or the same as the 87. I've found many wires present for various options (that were never installed), but sometimes you have to go back in the harness to the last connector to find them, sometimes not. It's likely the harness with the speaker wires runs behind the side panel, behind the spare tire in the cargo area, then up to the top and into the hatch (all of my XJ's have come with the rear speakers, I never needed to back trace the wires completely). While your in the hatch lubricate the lock and latch mechanisms and check the grounds for the rear wiper for rot, things tend to get crusty in there.
There are a lot of early XJ headliner threads out there, a google search for "XJ headliner" is likely to turn up some pictures.
I haven't a clue about the taillight covers.
I took off my, Euro type, full front brush guard and tossed it. I looked at how it was mounted and thought in any sort of front end collision it was likely to cause more damage, than do any good protecting anything.
