I didn't know back-up lights were a legal requirement -- oh well.

Anyway, if it's an auto, there's a neutral safety switch under the plastic cover beneath the shift lever. This switch can get out of adjustment as motor mounts and transmission mounts sag (and as the busings in the shift linkage wear). You might try adjusting this switch forward or backward, but the trick is not to get the neutral/park starter lockout out of allignment at the same time.

On my car, adjustment didn't work. The contacts inside for the reverse lights were burned away. I had to rig up a micro switch separate from the Volvo neutral switch. (I guess I could have ordered a new switch.) Check for this problem.

The next item is the backup relay. I don't know where it's located (sorry). If you can find it, you might check to see whether it's getting energized (or if there's voltage across the relay coil) when the shift lever is in "R". This might be the first thing to check before digging into the netural switch (assuming you already checked that there truly is no voltage getting to the backup bulbs).

-Adam