It sound like an electical problem. I would think it to be a defective switch, relay or wiring . Try pulling out the relay or fuse.
You won't hear the blower motor when you turn on the AC, and it won't blow air through the AC vents. If you want to open up the HVAC compartment, you can test it with a voltmeter.
Common causes, blower motor resistor, blower motor, wiring or controls
check heater blower motor blower wheel could be broken off The knocking comes from an Acctuator that has stripped out in the Hvac case. You Have to replace the acctuator to solve the problem.
A malfunctioning blower motor resistor will cause the blower motor not to work. When the resistor is bad the blower motors will not turn on.
I think that you might have a bad blower motor, but I'm not really sure. Here is a link to a "How To" video of replacing the blower motor--> http://www.youtube.com/watch?v=frZl49p2quk It is very easy to do and don't waste money taking it somewhere to get it installed.
When you beat the blower motor, you are probably shaking the blower resister. The resister is what controls the blower speeds and is probably bad or loose.
(1)If vehicle is equipped with airbags, disarm the airbag system. To do this, turn the ignition switch to OFF position. Detach the cable from the negative battery terminal, then detach the positive cable. Wait two minutes for the electronic module backup power supply to be depleted. To enable the system, turn the ignition switch to OFF position. Connect the positive cable first, then connect the negative. (2)Disconnect the blower motor connector. (3) Remove the blower motor mounting screws and remove the motor. (4)Remove the blower motor wheel hub pushnut and the clamp from the shaft. Pull the blower motor wheel from the shaft. (5) Installation is the reverse of these steps. - Mazda Pickups Repair Manual
blower motor actuator needs to be replaced.
What can happen when a blower motor resistor goes bad is that the motor will run and then suddenly stop running on a regular basis. Sometimes the blower fan will not turn on and cause the motor to overheat.
To move air, that is what it does.
The blower in most 90's Cadillac's indeed can be changed out however, you will need to cut along the lines as indicated in the plastic barrier found housing the blower motor itself. Odd as it is, you really do cut this plastic shield and remove the blower. I've done it twice and it works just fine. NOTE: It is not easy getting a new blower back in as it was getting the old one out. Just have a little patience, go slowly and turn the blower motor slowly as you put it in. (not the squirrel cage itself) but the motor housing. It seems to fit easier with a gentle turn as it drops into place.
when you turn on blower it makes a strong odor of burning rubber.