Sounds like shorting caused by bad earthing in the bulb sockets. If the bulbs have blackened, that will be water intrusion.

Check the seals and clean out the bulb holder before epraying them with WD40. Replace the bulbs and check for correct operation before replacng the covers. Replace blackened bulbs with new tail and stop bulbs, double filament. If the seals look suspect re seat them correctly and if unsure either apply some red rubber grease or silicone sealant.

I have assumed lights in the rear half bumpers but the same is true of the lights on the rear body posts. These leak whether dummy lights or plates. Unfortunately this goes unnoticed and the minute drainage holes at the bottom of the rear corner of the rear wing blocks allowing them to fill with water and silt. This soon causes the corner to rot out. I always remove mine, clean the muck out and then repar any corrosion before applying a liberal amount of waxoyl to the entire area and reset the dummy lights/plate using clear bathroom silicone sealant. Access to remove the two 10mm nuts that hold these in is gained by removing the rear plastic boxes on top of the rear arches and then removing the inner trim panel. It sounds a pain but it is well worth the effort and does prevent that lower corner from rusting. I did my 91 when it was 3 years old and to date not a sign of rust yet my 96, which I got last year has holed on both lower corners because of water and wet silt blocking the drainage holes & remaining wet. While you have the rear panels off Waxoyly the seal of the rear inner and outer wing. I flood that "crack" with Waxoyl again to prevent wheel arch joint corrosion.

Goodluck

OLDMINIMAN