Latest Update:
The damn thing works! (but then, Danny knew it would, eventually...)
I needed something to test it with after the circuit mod, so I decided to just use a big honkin' ol' 12V relay so you can hear the pulses. Remember, I'll be driving a solenoid (click-click-click) fuel pump, so a relay makes a lot of sense - it makes the same sound, at least, sitting on my desk.
So I changed the cap, jury-rigged a MOSFET to drive the relay/pump, hooked up the relay and got this:
THAT, my friends, is a heater fuel pump speed in the ballpark's infield of being "perfect"! I may have to play with it some more to just use the middle 2/3'rds of the range, but that's an easy change, too. I also have to decide where/how I'll mount the MOSFET (it's a solid-state relay for the pump) and make that look professional, and then get a nice little box to put it in for under the dash.
Now I'll be able to turn the heat up and down just like Danny and Jack Crosby.
I love it when something I do actually works the way I want....