A Line Mechanic reports he forgot to remove a lockout pin from a left engine thrust reverser control on a MD-82 aircraft; after he had re-placarded the same engine reverser light under a different MEL.

Narrative

I had changed the lower thrust reverser 'open' light switch on the left-hand engine. The MD-82 aircraft was on placard MEL 78-1; but after changing switch there was no help; the light stayed on in cockpit. So I assumed it would stay on MEL 78-1 and [I] left the pin in the thrust reverser control. After finding out the reverser light staying on in the cockpit; it had to be re-placarded under MEL 78-2. This MEL doesn't require for the thrust reverser control to be pinned (locked-out). [I] did talk to Maintenance Control to notify change of MEL; which was changed to MEL 78-2. Be advised that weather conditions were cold and raining 45-degrees and steady rain. Also checked reverser was stowed with a logbook entry.Two reasons why the event occurred: One; by me assuming that the MEL would stay on 78-1; I didn't bother to unlock the thrust reverser control and after dealing with closing-out the original pilot report and to re-placard it under MEL 78-2; I had forgot to pull the [lockout] pin. Second reason; when having the thrust reverser placarded; I had also assumed that the whole system would be inoperative. On MEL 78-2; the MEL does not state that the crew can still operate [the Thrust Reverser] System. Suggest [being] more thorough with the MEL procedures and tasks for [reverser] closure. Also; to go back for a second look on my MEL procedures to look for mistakes. I never had this combination of [changing] one MEL 78-1 to 78-2 before. I could assure this will never happen again. Reviewed MEL 78-2 and found my mistake.