Super Moderator. Motivational Poster
To test this theory, I'm going to leave the regulator out of the equation. I'll test the motor under full 12 volts out of plug. If what you're saying is the case, it should fail to run as soon as I load it. Motor runs beautifully on bench test.It looks to me that you have an issue with the 12V supply line to the regulator. The supply line shouldn't drop to zero or a very low voltage when you load it up, ie try to draw heaps of current to run the blower.
My next step would be to identify where the regulator supply line originates (maybe at a fuse or junction point) and measure the voltage at that point with and without the blower. It should stay close to 12V.
You have all the symptoms of a dry solder joint, damaged wire or corroded connection (connector?) in the supply line. All of these will produce 12V on no load but fail under heavy load.
Without checking my ACC documents the signal wire from the ACC controller seems to be doing the right thing.