Lower consumtion in 12V led strip
The other solution that are mentioned do work but in my opinion the optimum solution to get the current/brightness that you want is to use a regulated 12 V (or less if you want) no matter the car battery voltage is.

You can get such a proper regulated 12 V by using a boost/buck converter module. They're not expensive and here's an example.



Note the screw on the blue potmeter, you can use that to get 12 V (or whatever voltage you need) at the output.