First check if you have 12 volts on the dark green wire to the fan motor. If you do not, check for it on terminal 30 of the relay.
These diagrams are for the heater with AC. If you have just the heater system, I'll prepare different diagrams for you.
If you have 12 volts on the dark green wire, check also on the gray wire. That has to be done with the connector plugged in. The weakest link in the chain is the fan speed switch. It's common to find the connector terminals burned and melted, especially if the fan is used on the highest speed often. This is a good suspect if you have 12 volts on the gray wire.
Images (Click to enlarge)
Monday, September 21st, 2020 AT 8:23 PM