300d rear window problems.
 

I am having an issue with my rear windows. I swapped the front switches out with the rear ones and they operate the front windows fine. with a test light, the rear swtiches are toggling voltage to the motor, but the motor doesnt work. when i run a seperate 12volt source, the motors will go up and down. is there a common problem with voltage drop issuses to the rear motors or insufficient grounds? even though the child safety switch is bad, i have bypassed it , so that is not the issue. any suggestions? thanks a lot.