I worked 6 years in electro magnetic interference (EMI) suppression on military aircraft. Most of the issues that were encountered, (things turning on when they should not, or irratic componet behavior) were either a ground loop problem, or an emission from a radiated source.
As previously suggested, check chassis ground from battery for resistance, remove the battery and run the truck, try taking a fused wire and temporarily grounding componet return to chassis. Attemp to isolate the issue and see what you come up w/.
only a few suggestions to try.