Erick,
I'm not a car person. But since you said initially that it was a heat related problem, would it be a worthwhile thing to do to just apply the 12v (and earth) by means of a makeshift cable/connector, and operate the cluster out in the sun, or failing that, in the oven, to duplicate the heat. I'm not suggesting you should shove it in alongside tonights curry, but if the cluster stops working at sa fairly lowish ambient temp, it may help pinpoint the problem.
Daft as it may sound, I did actually actually manage to find a fault on a pcb years ago by heating it in the oven to find a heat related problem. Of course, you have to learn to work quite fast as it's a bit uncomfortable squishing up and working in such a confined space
PS - if this is a totally crap suggestion, my apols in advance
Cheers
