Jay, electronics exposed to an outside environment can end up as an adventure!
My previous diesel pickup was a 2006 GMC and the only repair issue I had with it was mouse induced. In 2013, I was getting ready to make a quick run to the post office before it closed and I noticed the check engine light was on before I even put in the ignition key. The truck started and operated normally but when I went to shut it off at the post office, it kept running even with the ignition off. When I got home, I pulled the ECM relay to kill the engine. The next morning I went out to troubleshoot it but it was operating normally and the only stored trouble code was for loss of communications between the engine controller and the separate controller for the Allison automatic.
A couple of week later it happened again and a few hours with the service manual and some measurements allowed me to run down the trouble. Most GM products use a UBEC (Underhood Bused Electrical Center) where the relays, fuses, and interconnecting wiring live. The top layer contains the relays, breakers, and fuses, the middle layer has open copper bus bar wiring used for interconnecting the various circuits, and the bottom layer has connectors for several wiring harnesses. Within the UBEC bus area are both constant and switched 12 volt lines and mine had leakage between the constant and switched 12 volt lines. The ECM was being fed 12 volts on its switched input via leakage and after a few minutes, the leakage charged its switched 12 volt input sufficiently that it turned on its relay. The loss of communications occurred because the Allison controller wasn't seeing a 12 volt feed until the ignition was on.
The root cause was a mouse that thought the UBEC made a good urinal. The mouse pee caused corrosion of the bare copper bus and the intermittent nature was because the corrosion was only sufficiently conductive when the humidity was high. Troubleshooting time was around three hours between studying the service manual to understand the logic of the system and making a few measurements. Replacement time was only a few minutes, the dealership didn't have one available but I found one pulled from a totaled identical model on ebay shipped for $50 and with several vehicles waiting 3 days for parts wasn't a problem.
Photos below are the UBEC, a crop of the damaged/corroded area, and the less than 1K leakage between the switched and constant 12 volt buses when humidity was high. I started using mouse traps in the detached garage after this event. The manufacturer used bare copper because of the multiple "stab" type connectors used in the UBEC construction but coated wire would have been more suited for the environment seen in outdoor use.
And on edit, using my Tektronix 851 for measuring resistance was test equipment overkill for this except I was curious to see how the resistance varied over time so I used the line operated Tektronix digital tester in its "simple" mode instead of a battery operated DMM.
Rodger
