What do you mean, don't seam to work. Either they work or they don't.
Ok, look in each socket for any corrosion where the bulb gets pushed into it.
If that looks clean, follow the wiring from each one, as far as you can and look for breaks.
If the wires look good, see if there's a fuse that controls all 4 markers and check if it's blown. Personally, I don't think there's one fuse just for the markers but you never know.
08 GT 5spd
Spectra CAI, Comp Cam Detroit Rockers, Kooks headers w/catted H-pipe, Revolution Automotive dyno tuned, FB430 race mufflers, 3.55s, JLT catch can, BMR Lca's and relo brackets. Prothane motor mounts and sway bar bushings.
20% tint, smoked markers and tail lights w/sequentials. Clear front signal covers, series 3 chin spoiler.