If the motor does run for a few seconds before the fuse blows, it is most likely the motor itself.
There's 2 ways to find out :
1. Replace the motor. ( swap it between the seats, lot's of work but you'll be able to clean up those 10 year old buggers that have been hiding under the seats. )
2. jump the fuse with a big *** wire and look for smoke : MOST DEFINITELY NOT RECOMMENDED !!!!