Although nearly impossible to replace, it is easy to see. Get a flash light and look through the center vent. The blades of the motor are visable.
In Florida we occasionally got one of these things going again by sticking a long (I mean long) thin screwdriver through the center ducts and rotated the fan blade while under power. I say Florida because this only works on motors that have stuck from lack of activity. If this happens to work leave it run for a good time before shutting down.
The electrical is exposed at a four pin plug on the firewall in the area of the brake booster (to the center of the car if I remember correctly). I have diagnosed the electrical hundreds of times over the years and never have I done it without a wiring diagram in hand. Its a goofy system with one blower switch running two different motors. The switchover takes place on a switch directly above the gas pedal. The switch is moved by the vacuum actuation of the mode door. I think the default position is heat though.
Bosch Master, ASE Master, L1
33 years MB technician