Well...its not a voltage thing. The cars electrical system regulates what voltage is supplied. Your problem is the current that is being drawn buy the LED lights. Its probably not enough. did you just buy the LEDs or are they made specifically for your car. If you have a multimeter you may be able to figure out what you need. Take the original bulb and read the resistance across the element in Ohms. Then read the LED resistance. Then just wire the appropriate number of LEDs in parralel to match the resistance of the original bulb. If that doesnt work.....hell i dont know!