Different LEDs are rated for different amounts of current. I do believe that if you do it right, you can get it brighter without messing around with anything else. My advice to you would be to, if you want to try things out before you do them to your car, make a little prototype outside of the car to match the current, etc. If it works, then go ahead and start working on the real deal. If not, then you can either keep messing with it till you get what you want, or just give up. Perhaps you should call up the manufacturer and find out what they suggest.