you running l.e.d.'s in that undertail?

if so, try switching the polarity of all the lights that aren't working. a regular light bulb will work the same with the polarity reversed, l.e.d.'s don't work that way.

also, if you run l.e.d. turn signals front AND rear, you may need a load equalizer. l.e.d.'s don't pull anywhere near the current that a regular bulb does, and they'll make the flasher flash very fast... sometimes so fast that it looks like they're just stuck on.