The seat has to be able to move back and forth in order to remove it!
There is no other way (except to cut it out) to remove a front power seat than to move it all the way back to get to the front hold down bolts and then move it all the way forward to get to the rear seat track hold down bolts. If the seat won't move you have a problem.
Problem & Solution
Common among these chassis:
W124
W126
W201
Watch Kent's free intro video to learn the basics then watch his indepth on demand video to learn all the tricks so you can do this yourself.