[dw1]

As with the manual heat in my '87, there are 5 things to check:

The heater switch in the center console.
The engine compartment relay.
The fuse (plug-in type) on the blower motor wiring
The blower motor itself
The resistor (very unlikely the problem but what the heck)

The problems with mine turned out to be mainly, but not entirely, the relay.

Because a replacement relay is almost impossible to get I opened it up and rewired it to eliminate the time delay portion of the circuit. The blower motors also needed a bit of lube. Since then the heating system been working great.

The air con in the cars may be marginal but a fully functioning heater will bake bread.