Since you've got power at the bulb sockets you should check the ground side. Just below the left taillight lens you'll find the grounding point for the back-up lights, as well as some other stuff. It's easy to spot. Look for several brown wires attached to the body by a single stud. It's a longshot, but it's quick and easy to check.

Also, have you checked the fuses at the headlight switch? I know you say you've got power to the taillight assemblies but it's a good idea to double-check yourself sometimes (believe me, I've learned this the hard way). You'll find these fuses behind the panel at the left side of the dash. The drivers' door must be open to gain access to this panel.

Lastly, the back-up lamps on your car aren't all that complicated. Between production dates 9/97 to 5/99 there is an actual switch for the back-up lights. It's part of what's called the Range Recognition Switch. It's located in the shifter assembly. The back-up lamp switch can't be replaced separately, but the Range Recognition Switch can. This switch is hard-wired to the headlight switch, where the fuse is, and then on to the back-up light bulbs.