I had the same problem with my wiper motor. Had it out of the truck and everything worked fine and I put it back under the dash board and the thing would barely move the linkage that connected to the wiper arm studs. I'm probably just gonna bite down really hard and buy an electric conversion. Unfortunately they're about 190 bucks from what I've seen.

When at first you don't succeed, get a larger hammer.

1953 Chevrolet 6400 Flatbed