...I had our "heating and air conditioning blower motor resistor: service and repair blower motor resistor replacement" done by GM on our 2500HD under recall (the truck was 7.7 years old when the "no charge work" was done!). This large electrical component was under the dash on passenger side inside the cab. Word is that this component was subject to in-cab humidity, and corrosion, even though it is not directly exposed to outside weathering elements.
I can imagine older trucks having (possibly) similar vulnerable under-dash electrical components that, over a decade or two, could degrade due to in-cab humidity (ie. your floor mats are wet over many winter months over many years, then bingo, intermittent electrical issues) ?
...now that you've isolated all the components on that 30A circuit causing a parasitic draw, you just need to isolate the single component, and replace it...
