sounds like there is no ground on the motor housing. one ground from the motor housing goes thru the shield of the us2 cable to the sonar where it ties to the head unit ground. The other ground is fed thru a 3a fuse in the tm top housing and goes to the tm battery ground.

Take an ohmmeter and read from the tm housing (clean connect to metal) and negative of tm battery. You should read near continuity. Continuity = 0 ohms. You should read some small amount of wire resistance.