It almost sounds as if you are running them without enough resistance. Look for a data sheet for them if you know make/model. I know that the proper formula for the resistance is on the site somewhere. It looks something like:
R = V - Vf * 1000 / mA
R = Needed Resistance
V = Voltage (car)
Vf = Forward Voltage of LED
mA = LED's current draw
I cant remember if that is the correct formula, so dont use it unless you can double check it with someone else.
