Same exact thing as mine...including the inside/outside air flap. Except my "thumping" wasn't as fruitful. Hitting a chuck hole the other day brought it back to life though...too bad I was in sight of the house on the way home....but it didn't come back on later.
I'm thinking an intermittent connection somewhere between the blower, the relay and the ground or 12vdc. Like yours, the relay and switch are working fine. Swapped the headlight relay for the blower and the lights still worked! The relay clicks as expected when the switch is moved.
I hope that someone takes pity on us and responds with an easy resolution. Don't really want to go the route of removing most of the obstacles between the air intake and the blower under the hood. Its going to be single digits to teens tomorrow. I have to keep a window cracked to get enough airflow over the windshield or my feet.
I'll shoot back a response if I come across a "fix."
1994 - 265,000+ miles and praising God!