passenger side brake light keeps going out. on 2000 Mitsubishi Galant

brake light continuously goes out, it comes back on once i mess with it but then turns off shortly after. im guessing its a short but how do i locate the source? any suggestions would be appreciated thank you for help.

If it goes out it's an 'open' if it blows a fuse or burns wires it's a short (very general) you can have ground-to-ground shorts in ground controled circuits but they generally cause feedback and wacky problems not described here.If the light comes on simply by lightly tapping the housing the filament is most likely broken and when you 'mess' with it the filament makes contact and works , as you've noticed , temporarily.If this sounds like what you've got first thing is to put a known good bulb(poss. brake bulb from drivers side) if symptom switches sides you know it's the bulb.If not start checking the wiring harness especially where it runs along the hinge pull back or unwrap any neccessary to make sure no broken but barely touching wires.