[LMP]

Probably a wiring problem, like a corroded ground point or splice and this is not unusual after several years and water/salt intrusion. THis is hard to pinpoint. IF this is the case, it is a trial and error operation. I'd try to locate any wire that represents a "ground" (there are several that run across the vehicle) and try to measure actual voltage between some point on the wire and body sheetmetal. I use a needle to pierce through the insulation (of ground wires at different places) and connect to the voltmeter. THen run various accessories with engine either running or not and see if a voltage shows. There should be 0 volt or barely a few tenths of a volt. Of course this is tedious and can lead to nowhere after several attempts, but you can be lucky too.
There is a hard ground point near the starter. Locate also and test ground points near the computer unit, a senbsitive area. ...