strange tail light problems

hi everyone,

I have a 1989 chevrolet k 1500 4x4 truck.  Had some work done on the
box recently (it was removed) and when i got it back my tail lights
havent been working properly.  I have both running lights, when i hit
the brakes, the driver side light works but the reverse lights come on
(only 1 of them), the other side has no brake light at all.  When i
put it in reverse, no reverse lights work at all lol.  I checked the
wiring underneath.  everything appears to be wired up (none loose or
anyhting) and the ground appears to be ok.  any ideas what the hell is
causing this to happen??