The flipper button uses an optical switch / flipper onto board which runs on 12v. The flipper buttons on the handle don't.
Since I'm assuming your optos work and your other flipper works, sounds like a flipper board issue, or some screwy power going to the flipper board because of the mod.
Try checking your voltages at the flipper opto board, compare it to what you are getting on the working flipper opto board.
Try swapping the left and right flipper opto boards.
Games were't designed to run mods, they were designed to do exactly what they are doing. Sometimes the game can handle it, sometimes they can't. Typically, a game can handle it, but installing mods can bring issues out of the woodwork that you never would have noticed if you didn't install the mod.
Quoted from Benfica99:
I got myself some under cab led-strips from Tim Mezel, and i really like 'm. But a weird thing happens on my Demolition Man: when it's plugged in (12v, left of the coin door ), the right flipper loses its power when it's triggered by the flipper button- no problem with the left one.
But i can't figure out why there's no power drain when i use the gun-handle to trigger the flipper...then it works just fine. Anyone has a suggestion?