so, its possible the rheostat has failed internally.

man.... i don't know why this post didn't show up in my notifications sooner.. more than likely to late, but, in my case, it was literally the ground i ran from the tac itself.

in your case, the rheostat is literally a ground. the dash illumination lights cannot themselves be grounded, as pointed out in that thread you linked to. as again, the rheostat is a 'floating' ground, itself is the ground. if you run a hard ground/ground to chassis it cause all kinds of issues.