Hi - Cal, John's assessment is that it is most likely a conducted noise problem, not a radiated noise problem.  This seems very likely. If that is the case, the tape only complicates the issue, depending on where it is grounded.

As a start, I believe it would be worthwhile to draw out a wiring diagram of everything you have, including the existing ground system and how you derive the 12 Volts from the higher voltage for the motors.  That  diagram can be valuable for future maintenance as well as showing common paths creating the noise problem. 

Good luck, Bob