bench test pw motor

[kglowacky]

1978 Silverado I just bench tested my pw motor. You test by connecting one to positive the other to neg. reverse them to reverse motor . I think I got this figured out BUT not really. The way my 78 works is with 3 wires off the switch. Most I have seen on utube have four. This lies the issue. With the four wire the 4th wire is a ground and it fills the circuit of the opposite side of the switch with a ground so when you push the switch the switch lets power to one side and ground to the other. My problem is I dont have that 4th ground wire, So when I push the switch I get power to one side but no ground to the other and when I push the switch the other way the power goes to the other side but no ground. I do have a ground wire coming out of each door harness ( I grounded to each door frame)and one that connects to the firewall area but can't figure how the motor gets grounded. Thought it was the switch but I tested for ground on the front side with no good results. Anyone know how these three wire switches get ground to the motor? Thanks