Gain access to the blower compartment

Look at the blower motor and you will see a red and blue wire connected to it. Jumper the BLUE wire to ground (Yes, it will spark!) If the blower is good it will start to run at full speed.

The regulator works by varying resistance to ground, so this test effectively bypasses the regulator. BTW, this test will work with the key on or off because the red wire is hot all the time..

Edit: Found the DIY with pictures: http://www.peachparts.com/Wikka/W140BlowerMotor

Regulators aren't too cheap, but they're an easy DIY to replace.