Constant heat

Hello all, here is what I have. 1995 Blazer LT 4 door, 4.3L W code engine.
This is a friends vehicle that she just bought and I replaced cpi, nut kit, and fuel pump to get it started. Vehicle was bought knowing it did not start. Runs great know thanks to all the help from you guys in this forum. But now she can not change the temp control. Only has heat constantly even when set to cold. All vent controls operate like they should and the blower operates on all speeds. Thanks for any help.

I've never dealt with this problem personally, but it sounds like the "vent door" may be stuck open. I would run a search on this site for that...

The blending door is stuck or the the temperature dial has become defective.

Oops- my bad, yes it's the blending door.