1. Remove the back bulbs, [*]Look at the filiments [*]CHeck 'em with an Ohmmeter [*]Filiments can fail, and fall accross one another [*]This makes a "cross circuit" possible [*]And Drives owners nuts.
2. Clean out the sockets,
3. I favor Carb Spray,
4. Inspect the inside of the socket,
5. Make sure it's not corroded,
6. Look at the bulb base and buttons,
7. Make sure they are not corroded,
8. Use some Spark Plug Dielectric Grease,
9. And cover the base of the bulb on re-install,
10. Make sure you re-install correctly.
11. And try it again.
12. Did the problem start suddenly,
13. Or were the symptoms occurring over time,
14. And ignored?
15. But I still favor Taz's answer!