Chevrolet Repair: Heater controls, resistor pack, blower motor

Question
Hello,
I have a late 88 Chevy PU. The problem is every winter the heater only works on low. I can't get it to go on medium or high. I have already replaced the resistor and the control head and the blower motor is working fine. It seems to work fine on all speeds in the summer when the AC is on. I have asked many people what the problem could be and can't seem to find an answer...can you help?

Thanks!

Answer
Hi Floyd,
Sorry this took so long. I had an emergency.
There are two relays, and sounds like the high speed relay isn't working correctly.
You can swap them, and see if something changes. They should be exactly alike.
Look for them beside the resistor pack behind the glove compartmant.

Van