[Hutch]

hammered68
There is a relay behind the Glove Box up high on the firewall very close to the passenger side cowl (Near the Antennae) that has a short black ground wire with a ring connecter that is supposed to go to frame ground.
When Folk's work on the heater cores them either does not attach it correctly or not at all.
This can be tested with a short (2 Ft.) length of wire.
Turn the Fan on High and attach one end of the wire to ground and then to the Black wire
The black wire is supposed to be under the stud nearby that holds the air box together.
This may not be the issue but this is what I found when my high speed on the fan was not operating on High Speed