Paul,

Find the cable coming from the blower motor/regulator. I think it is between the brake booster and the center of the car. I can't remember if it is in front of or behind the false firewall but it is somewhere around there. Three wires Brown, Black/Green, and White/Pink. With every thing connected, engine running, you should see 12v+ between the B/G and Br wires. Then check the voltage on the W/P wire to the Br wire. It should be between 1 and 5 volts or more, not sure. 1V is low, 5V is high and it should vary depending on the speed called for by the control unit inside the car. If this voltage varies then the regulator is probably bad. If doesn't vary then the control unit is probably bad.
Most of this is from memory so take it with a grain of salt and forgive me if I forgot anything.

PaulM

__________________
84 500 SEL (307,xxx miles)