The problem with my 300td's wiper assembly was physical destruction of linkages and the aluminum casting due to multiple screws backing out. Thankfully, I was able to cobble together a good one after obtaining another damaged one from a junkyard car.

I'd continue disassembly until you find what's binding the motor. Maybe there's a loose screw jamming the works. I still have some parts in the corner of the garage, so if you find something that's broken, maybe it's OK on what I have left.