I'm curious also. My left turn signal started clicking twice the normal speed and then I found the left side back up light was burned out. First I'd bout the flasher relay because back in the seventies if your turn signal started acting up it was always the culprit. Sometimes I forget what year it is. lol Anyway I changed all of the back end bulbs and now neither back up light are working. I used Philips long-life for the brake and turn signal bulbs and Bosch long life for the back up bulbs. Naturally I can't help but wonder if I should have shelled out the extra twenty bucks for the Philips b.u. bulbs.
I just couldn't understand went those little bulbs cost so much.