[mrein3]

It has been a while but a couple of years ago I swapped out my non-AC heater stuff for AC. The plug for the fan switch I got from my used parts hunt was slightly melted and some of the wires messed up. When junk yarding for another harness I discoverd most of the AC switch plugs are melted up the same way.

So I rebuilt what I had. I ordered a new switch, new plastic switch plug, got some wire ends at NAPA, and some colored wire to match the wire diagram.

REALLY long story short, there is a set of one, two, or three resistors after the switch to step down the amount of voltage running to the fan. This controls fan speed.

I can't remember what the numbers were, and you didn't specify if you had AC or non-AC, but I'm thinking I had 3V, 6V, 12V, and 12V. One of the 12s turned on a relay that really spins the fan.

Regarding the heater control valve - they do go bad. If yours is original I'd just replace it and move on.
However I think they default to open. In other words when they are broke they are open. Maybe somebody else will chime in here on that one.