So, my heater has never worked very well, but it's not a winter vehicle (92 Geo Tracker 1.6l 8v soft top) so I haven't cared much.
However, one day the blower motor just stopped. One thing led to another, and a new blower motor + resistor fixed that.
BUT: the heater controls are messed up. I think someone tried to fix a blockage (I hear that leaves building up in the fan box is quite common? - I did find some in there), and incorrectly connected the slider controls behind the dash. It's difficult to describe the results, but let's just say I'm pretty sure they connected the wrong