Bizarre Electrical behaviors
The bullet-style fuses get a corrosion on them called "tin pest", which is non-conductive. You can get copper-coated ones from AutiHaus AZ or from FCP Groton very cheap, that do not go cruddy on you. Clean the spring clip connectors with Emery Cloth to get the pset off. Use Ox-Gard paste to form an oxygen-proof barrier around the connector that will not interfere with electrical conduction, but will inhibit any further corrosion. There are two relays that can start Inductive-kick transients onto the cab interior bus when the diode inside the relay goes bad (surge suppressor diodes are there to shunt inducive kick). One is the timer for the chime, the other is the central lock relay, and ALSO: The "Main Relay" can do the same thing to your fuel delivery circuit, which will eventually toast the O/P drivers on your ECU to the Injectors. There are only four kinds of relays in the whole wire harness. Order them by Bosch number from BenzBin.com or FCP Groton, and they run $5 to 11 apiece. EZ way: Spend $60 and Swap Out Relays until the problem disappears, or NOT EZ Way to save $40 of that 60...follow the schematic printed right on the side of the relay can itself, and measure Fwd-vs-Reverse bias across the surge diode(Fwd Bias should be @ 2K Ohms for a Bosch 1215613 relay, Reverse should be >2 Meg Ohms) A bad surge diode will read same both ways: Diode Symbol is an arrow. Connect Multimeter on Ohms function to relay terminal of a REMOVED relay so that the red lead is upstream of the arrow, and the black lead is connected to the relay terminal indicated as being downstream of the arrow. That makes up connections for a check of Fwd-Bias. Switch lead locations to check reverse bias. Replace relay if bad or screwy readings.
Good Luck to you, Bloke.