Matt, please let us know if grounding was the problem. I too have this problem on my '88 420 SEL and it was preceded by a problem with my rear left sidemarker lamp that would not light. When I opened the rear tail light assembly the side marker socket had a 21W bulb installed instead of a 10W bulb. I installed a new 10W bulb and the side marker still would not light although the new bulb was fine when I tested it on the right rear side marker. Then I wake up this a.m. and the left turn signal flashes rapidly. No warning lights lit on the dashboard. Fuses are fine. Thanks.