Since you have better access to the components in the dash at the moment, checking the wiring to the Automatic Heater A/C Control Unit would likely be the best indicator at this point.
Find out which wires have constant voltage being supplied then we can narrow it down between which wires/inputs SHOULD actually have voltage being supplied with the ignition switch off and which wires/inputs are possibly getting a supply of voltage at the moment that SHOULD NOT.
If we know that and it's determined that there is voltage currently where there shouldn't be, maybe we can start tracing backwards from there and determine the source of the problem.