So I have no heat in my '85 QV Cabrio. Being familiar with older Mercedes, I pulled the heater valve expecting the rubber plunger to be damaged. Good news (I guess) is that the valve is in great shape. I tested the rheostat as listed in this post (https://www.ferrarichat.com/forum/threads/mondial-heater-not-staying-hot-please-advise.84386/) and am getting constant power regardless of the position of the heater knob. Sweeping from cold-to-hot has no change in the light. So it seems the problem is electrical and I'm not sure where to look next - or how to best test the rheostat to make sure it's functioning. I do have fully working gauge lights and the fan speed works fine. It's only the actual heater that's not working. Any ideas what to do next?