Hi Frank,
If you are using a 12v to 6v converter (commonly known in the electronic world as a "Buck Converter") then there will be an input for + and - incoming voltage and an output for + and - voltage also. Most of these will be adjustable, but you may find some which are not adjustable. Please look at this one:
https://tinyurl.com/2s4hzb6y You will see In+ and Out+ and if you could see the other side, you will see In- and Out-. And then there will be a small blue rectangular cube with a tiny brass screw on top...this is your adjustment device. So in this case, you could take any DC voltage from 6.5v to 60v and convert it into an output DC voltage of 1.5v to 30v. Please tell me...is this similar to the converter you wish to use? This is the best way to convert 12v DC to 6v DC and not be concerned with the 12v ground. Please tell me if you have questions. You will need a converter which has a high amperage output for your wiper motor. This one (10 Amps) should be okay.
