W124 Blower Regulator Repair???

[zeiglerr]

I replaced my blower motor brushes and I can make the motor spin now by grounding the blue wire, so it looks like the regulator is shot. The wiring diagram shows a "black box" with a big transistor. Could I be lucky enough that it's a big power transistor (that I might be able to replace) or is it a big potted mess that can't be fixed?? Anybody ever had one apart?

Into my 2nd week of having an "off road vehicle"