Hello Paul,
Not easy to work on electric windows or anything electrical without a meter and a knowledge and experience of how to use.
I think that if this was mine, I would be investing in a hour of a trusted auto electrician's time to at least get an experienced diagnosis on this.
Kind regards,
Gareth.
Hello Paul,
Before moving into the mechanics of this, have you measured the actual voltage at the motors to check that they are receiving at least the 12v. as they should be?
Kind regards,
Gareth.