Mechanic worked two aircraft with similar APU problems that required trouble shooting and repair involving the APU door actuator. The mechanic disconnected a cannon plug in order to trouble shoot an actuator and subsequently forgot to reconnect it.

Narrative

There were two APU problems that required troubleshooting and repair that night. It turned out that both planes required an APU door actuator. There were only two in stock in ZZZ. I removed and replaced the APU door actuator on aircraft 'X'; a CRJ-200 in the hanger; rigged the position sensor and started the APU in accordance with maintenance instructions. The aircraft was subsequently pushed out to the ramp. The other airplane; aircraft 'Y'; was then tasked to me because after a new actuator; ECU (Electronic Control Unit) and open and close relays were installed; the problem persisted.The APU door would open; but would not close and the [circuit] breaker would pop when you try to close it. Suspecting that the actuator was bad from stock; I decided to test the resistance values of the motor in the actuator. I initially tested the actuator I suspected was faulty. I then tested the actuator on the plane at the next closest gate. I then tested the actuator I knew was good that I had previously installed on aircraft 'X'. I reviewed my findings with my co-worker and decided it was a bad actuator. Since we did not have any more actuators; the problem was MEL'd and I was done. I neglected however to reconnect the cannon plug to the actuator on aircraft 'X' after testing the actuator.I need to do a thorough Operational Check of any system after disconnecting any plug or component. Take the time to perform that extra step to avoid any instances from occurring in the future.