Why don't my blower motor don't work

Your testing device may very well show 12 volts - however if that same circuit were put under a load - I'll bet it is much - much less. IF you hooked up a 12 volt electrical component - say a headlight bulb - it probably will not work either. I can't post the diagnostic flow chart that is needed to insert some method to the madness but that is what you should have. Otherwise it's a guessing game and that usually results in wasted time and money by installing parts unnecessarily.