Just because the car has a 12v battery doesn't mean the lights are operated by 12v. Figure out what voltage and amps each light bulb requires to run.
I believe I found the circuitry behind the light housing:
https://www.ecstuning.com/BMW-E82-13...ghts/ES207350/
This should give a good understanding on what is required to get this done. It's a simple concept to do but you don't want to take any shortcuts or cheesedick anything when it comes to power.