1991 Deville ... Instrument Cluster +
 

Problem... Instrument cluster went out along with heater/ac control/ fuel and mileage status displays... Sometimes I would get a very faded led display but then it would go away after a few seconds... now I get nothing..

Attempts: Tried switching out Instrument Cluster (Speed / mileage etc), no change.
Tried changing computer module.. That brought back a weak display for a few seconds, but then lost any display..

I am an electronic tech but not an automotive one. I suspect heater control module is loading down power supply, but I don't know how to get to that, to try disconnecting to see if problem clears or to trouble shoot voltage levels under load with boards connected and then unconnected to isolate problem. I am afraid I am blowing the ECM Computer module and the 2nd one is bad because the weak temporary display on the instrument cluster reappeared when I put a new module in, but then no display again. Fuses are ok. So I don't know whether I am not getting voltage out of a power supply somewhere, or one of the modules I haven't replaced is bad and loading down the voltage. Where is the module I am looking for and how do I get to it, and/or does this make sense to anyone that knows what they are doing?