Reverse Light help

My reverse lights no longer come on. I've replaced the bulbs and no change. I think something isn't engaging when I put the car in reverse. I remember someone having a similar issue but after searching the forum I can't find that thread. :dunno:

Any advice or help is much apperciated! Thank you!

EDIT: I drive a 2005 Tib GT 5-Spd. Ive added new deck/speakers + Sub and amp. Thats the only electrical work I've done thus far.