I just got done repairing an issue with my window amp. It was posted on here years ago how to fix the problem of an Intermittent drivers side window on an S13 240sx. I took a picture of the broken terminal that needs to be soldered. I'm sure that others could break so it's just a general tech on this stupid amp.
The amp is located under the door panel on the driver side door. It's a white and black plastic box with a wire plug coming out of it. Not too hard to spot. Once it is removed you can pry around the side with the terminals to free the PCB from the case.
Here is mine with the broken solder joint circled. Yours may be another one but take a look at all of the joints.
No after photo as it's pretty simple, just heat it up and add some extra solder to reinforce it.
Hope this helps,
Syn