On 2014-02-01, Gunner Asch wrote:
On Fri, 31 Jan 2014 22:38:43 -0600, Ignoramus16088
wrote:

We have a 1985' Ford LNT9000 semi tractor. Its heater used to work. It
was switched by a four position heater switch. It recently stopped
working. I just started troubleshooting this problem, and found it to
be very painful to do, due to cramped panel and complicated wiring
that is hard to access.

So far, I have been able to locate two wires that supply electricity
to the heater fan. If I apply 12v to them, the fan runs vigorously.

The truck, as far as I can see, has no fuses.

It has fuses. You need to find them

It may..may have breakers..but it has protection of one sort or
another

Now what does that "four position heater switch" do? Off,
low/medium/high?

Yes, this is what it does.

i