windshield washer motor

I`m trying to get my electrical washer motor to work on my Barracuda -67.
But it only runs when I jump a wire from the motor housing to negative ground on the car, and then push the washer button on the dash.
Does any of you guys know how the motor nomally gets its negative ground?
I did check in my service manual, but it didn`t say anything about it.
Many thanks in advance
Len