I have a similar problem with my heater in my '90 Classic. The 2 slower speeds are not working. Full power is. There are 3 resistors, 0.5 ohm, 1.5 ohm and 2.0 ohm to control the speeds.
I will have to take most of the dash apart to get to the resistors. What I have found, this is a 2 day job. Worse is to find new resistors. There is a solution where the resistors are not put in the motor housing, but where you can get at them easily. You will anyway have to take everything apart to install it.
ad a chance to look at her today, the 2 fans in the right footwell spin, only at low speed. I can't see the 3rdf fan but i dont hear ir coming on. the 2nd relay from the front is missing, i tink that was the heater/ vent relay, tried jumping it out but to no avail. I have a missing switch, looked to be the 4 position switch on the left, to energise ac, cant find it. the vent duct is seized open.
Where are the resistors located? I dont think the 3rd fan is bad yet, just cant find a relay 5 that switches between the 2 fans and the third ones, non have the same colors.
Its under the decker/cowl, pass side. Its prone to failure. Take your plastic grills out and have a look at it you should be able to see if it looks good. If you have to replace it hopefully you can get it out through the grill holes. If not you have to remove the hood in order to get the decker panel off.
while under there,I checked the fan, seized. got it freed up a bit, with generous appllication of oil with my telescopic oiler, even had the motor run, but then I'm guessing i must have hit the brushes as it stopped altogether. looks like i'll be in there to pull the motor out after all. Somebody did some work, found some duct tape in the dash. I'm seeing 16 Mohms, sounds like a layer of film on the brushes.
A nice big picture of what the motor looks like would be nice, searched the net for an hour.