led brightness reduction

I'm looking to purchase some LED tail lights from eBay for my car, however some of the tail lights I've seen recently from eBay have the low intensity running prety bright with the high/brake barely being noticeable when it is applied. I would like to add a resistor in the line before the harness to slightly reduce the brightness of the running lights so the brakes will be more noticeable. The tail lights I'm looking at have (I believe I counted) 42 LEDs in the assembly, that I'm assuming are around 3.2-3.8v each. Can anyone help? Btw they are red LEDs.

Dimming LEDs is much harder than dimming an incandescent bulb. You are going to have to experiment with resistor values.

Are there any values you can give me to at least ball park so I can kinda know which direction to go in? I have a bunch of little 12 volt 470 ohm resistors that came with little 5mm individual LEDs but I assume these are too small. Would I match the resistor wattage to that of my incandescent bulbs to start with and then try bigger resistance ratings? Or am I completely off? If it helps at all, it's an 02 Grand Am an the tail lights are (2) 3157 bulbs stock.

For safety, I would recommend using a 3 watt or larger resistor. Try 100, then 470, then 1K. Let me know if any of them dim it.

Sorry to bring up an old thread, but how about a voltage regulator? Or does current control brightness?

Current. PWM is the best way. Using a PIC or 555 PWM circuit is the common way. EG - for combined stop/tail etc LEDs, no PWM for stop and PWM at 20% - 50% for tail.