I am working under the presumption that, with the wire unplugged / plugged in, the OP is measuring across the harness that connects to the motor , is cycling the switch and observing a reading on his test equipment.

I will give that his statement " but when I connect it I get nothing. " is a bit vague and the " I get nothing " could mean no motor operation. Given he is testing for voltage, I take it as the voltage goes away when the motor it plugged in and switch operated.

Applying + 12V ( Or - 12V though I don't ever recall seeing this ) to both sides of the motor when in a non operational state then lifting one and grounding it when in operation performs 2 functions.

First it makes switches slightly less complicated because only one circuit needs to change state for a function.

The second less obvious function is shorting motor leads just after dropping the switch stalls the motor to prevent coasting / overshoot. As a motor is coasting down, it is acting as a generator so applying an electrical load creates mechanical load.

I'd apply power / ground directly to the motor terminals and see if it operates, reverse polarity to change direction as well . Use long wires so you don't get caught in the linkage and just brush the final connection in case there is a dead short. A fused / circuit breaker in the test leads is a god idea but sometimes we cheat.