What trolling motor do you have? On some, running the additional ground wire can cause a transducer and/or the unit to short out. "fixes" for interference are not all generic. The interference comes from the RF noise generated by the PWM board and the turning of the armature's against the brushes. Shunting the ground has been a way to try and eliminate the noise, but some trolling motors have that already done internally and trying to duplicate it on the outside could cause a short.
John