convert analog voltage to pwm
I assume you are looking for something off the shelf and not looking to construct a circuit from scratch. 

Fan speed controllers are commercially available that can drive 12v led bulbs instead of fans.  I would recommend using something like this:

https://www.amazon.com/Pixnor-Controller-Adjustable-Reversible-reversing/dp/B01H1W79S0/ref=pd_sim_60_7?_encoding=UTF8&amp;psc=1&amp;refRID=JX2HZEZDTSYRJ3TG651Q

And replacing the existing potentiometer with the one mounted on the board.  It would also be possible to use the existing potentiometer (assuming it is a 3 terminal potentiometer not a 2 terminal rheostat) by either disconnecting it from all current wiring and wiring it to the board, or using a resistive divider to change the output voltage range of on the orange wire to what the board expects.  If you want to go this way, post a follow up and I will post instructions on how to figure out what resistors to use.

Note that you don't want to use a fan speed controller rated for 12 or 15v because the voltages on car electrical systems can exceed the nominal 13.5V the alternator/generator outputs significantly when the engine is turned off.