Does anyone know how the RMP cluster wiring is set up. My 06 Ford F250 5.4 L RMP drops after I turn on my defrost or heating unit. It never drops in summer when I turn on my AC. Two things happen when I drive after 15 minutes of high heat. One, when I hit some bumps the needle will drop to zero. Also when I turn off the heating unit, the needle will also drop.

Where should I start to look for a lose wire under the truck or dash. Again if anyone has a diagram I could check for a lose wire or a bad ground.

One more thought. Is the the heating unit on the dash faulty?

Thanks,