The other day, I was driving my 1977 Chev Suburban and All of a sudden my blower motor stopped and my alternator light came on. The blower wouldnt work on any speed.
I have an aftermarket volt meter.

At the end of the day I started the truck again and the voltmeter read 12 and the blower still wasnt working.
I started out down the road to get the truck home, and when the engine got to a certain RPM the voltmeter shot up to 14 and the alternator light came back on.

I shut off the truck a couple times and same thing happened. Gets to a certain RPM and the volts go to 14 and the light comes on.

A couple hundred yards down the road the blower came back on and the volts were back to normal.
Yesterday I was driving and the same thing happened again.
This time I got out my spare alternator that came with the truck and put that in.
After that it didnt matter what RPM the engine was at the voltmeter stays at 12.

Checked fuses, all good.

HELP!! I have no clue even where to start.