A Line Mechanic reported a chain of events that contributed to his not properly connecting both engine N2 indicators while reinstalling the P1-2 forward instrument panels that resulted in a gate return for a B737-300 aircraft. Fatigue from an extended overtime shift; complacency; stress and distraction from concerns about meeting departure schedules were noted.

Narrative

After repairing a B737-300's L-1 (left) window for an overheat light issue and returning the aircraft to service; the next outbound flight crew at pushback stated that the L-1 window overheat light came on again. I verified that it had again failed and proceeded at the gate to swap the L-1 window to the spare sensor [electrical] posts. During this act of maintenance; I had dropped a screwdriver behind the instrument panel. In order to gain access to the area where it fell; I was required to remove the P1-2 [forward] panel.With the window now working normally; satisfied that it would cause no more problems; along with retrieving the screwdriver; I proceeded to install all of the removed components. I had notified Maintenance Control that I was confident that the window was repaired and that the aircraft could be ready in an hour to an hour and a half. No more than 15 minutes later a flight crew shows up at the aircraft stating that the aircraft was due out in an additional 15 minutes. With this additional pressure I started to make mistakes. First; while installing the glare shield; I thought that I had installed the instrument background lighting connectors; however as the gate return stated; I had not.Secondly; I was very cautious while reinstalling all the cannon plugs on the P1-2 panel; inspecting the marker tags two or three times before installing them on their related indicators. As you can see by the second gate return; I had misconnected the N2 indicators. All of these actions on my part were done according to the appropriate Maintenance manual and/or standard wiring practices. Now as far as some of the existing human factors playing on all of this; they are as follows:Fatigue [was] the number one contributing factor. I had not had any good rest prior to the start of my first overtime shift which was then extended to work on aircraft.Second factor was stress. When I had informed Maintenance Control that the aircraft could be ready in an hour to hour and a half and then the flight crew shows up in 15 minutes; I had put pressure on myself to complete the repairs so that a new on-time departure could be made.Third and finally; was complacency. I was certain that I could complete the repairs with no mistakes; I found that this was not going to happen. I can't express the way this made me feel. I am normally not this complacent with any part of my life; especially here on aircraft. The safety of our customer is our number one priority.