Maybe you just need to add some resistors.
You could figure out what size you need with this formula:
http://en.wikipedia.org/wiki/Resistor
Or you could do trial and error. I imagine an auto parts or auto sound store would have everything you need to get your LED's working properly.
Do you have pics of what you've done? I'm interested in how it looks - LED's rock! Very modern appearance and look much better than standard filament bulbs for aux lighting like brake lights and turn signals.
Good Luck,
Larez2