http://www.justanswer.com/uploads/CT...8_relays_1.png
You want the normally closed version (auto part store will have one).
1 - goes to reverse positive
2 - to ground
3 - parking light signal wire that you currently have wired to fog light
4 - wire from 4 to parking light where you current have parking light signal wire
So when the coil is activated from the reverse signal, the switch opens and stops sending power to the fog light.
Is this easy to follow?