I concur with checking grounds and battery leads. Saw a similar issue with my father's '91 Fuso, had a frayed wire grounding out to the chassis but there was enough grease and road gunk to give enough electrical resistance to not pop any fuses. Had turn signals, but hazards and headlamps would cause relays to buzz (resonating between on and off), it would start and run fine, and would occasionally lose memory/clock on the stereo.

I also would check the main feeds if you haven't already. Especially the fusible link box that gets corroded (had this on my '95 MM). Automotive electrical issues are always a headache since completely random and unrelated items will ground out to complete a circuit. Lots of "fun" to troubleshoot.