ok so im going to run a 3rd brake light (spoiler led assembly -35 led strip) off my 12v brake light, but i was told that the 12v is to much power and will eventually blow the leds. so what type of resistor should i put inline to prevent this?

 

What voltage are the led's. You would need more information than you have provided to answer your question. Like what voltage does each LED require? Also how much current does each LED require? If you doen't know the current at least find out what voltage they require. From their you could use an op-amp as a regulator.

From what I understand most spoiler LED's are designed to run off 12 volts.

 

yes, they are designed for it.

They either use enough diodes to not have any issues, or the LED's themselves already have the resistors to cut the voltage down. You should be fine as is