Some wiper motors will have the case grounded, which will be a problem when attaching to a metal frame. If the motors are run in opposite directions it will short out your power supply.
I spent the morning trying to see how difficult it would to isolate the ground. Fortunately it's not too hard. There are lots of different wiper motors out there but this is how my particular model looks. First chect to see if you're lucky enough to have a motor that's not grounded in the first place.
1. Remove the motor case.
2. Remove the gear cover. Mine simply snapped on and off.
3. Before I could remove the rotor, I had to remove the C clip on the worm shaft.
4. The metal screw eyelets are where the motor is grounded to the case. There is one spot on top (indicated by the circle) and one underneath.
5. On the back of the board I used a Dremel with an abrasive wheel to cut the traces to the grounded eyelet.
6. On the top I had an over temp/current device that I also cut the trace to.
7. I've replaced the ground with bits of insulated wire.
8. Replace the circuit board. I've removed the over current device on mine. I decided I didn't need it.
9. I had to tie back the brushes with thin bits of wire before I could replace the rotor.
10. Replace the housing and gear cover. A few drops of light machine oil on the bearings or bushings would be a good idea. Also check for damage to the geartrain. Mind had nylon gears. Be sure there's ample grease in the gearbox.
Finally make sure you don't have continuity between the motor leads and the case. On my gear cover, there was a tab to ground the common motor lead to the case. I removed it.
Enjoy,
Dave

Sydney Australia
and as I was in the mud.
I modified just A little bit more
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