I had mentioned that I put some led's in the tail lights in my truck and didn't notice the reply till later.... I went with a 6v 1157 style from amazon.. they were cheap, but they are far brighter than the incandescent bulbs, they are visible in daylight actually....
If these hadn't worked out, I was thinking of making my own with a 3.3 volt regulator (LD33V) and raw individual led's (I have a ton of super bright smd led's from a scout flashlight project that are left over) .. I still may try that, this way would work with 12v or if I keep it 6v...