I have been adding LEDs all over in my car and lost cruise control functionality. I understand it is because the LED is a diode and either doesn't complete the circuit or show enough voltage drop. I am looking for any neat solutions without going back to incandescent bulbs.
I started out with cheap led projectors on the tail brake bulb and third brake light but the third brake light lit up on its own with lights on. Then I tried some can bus led brake tail bulbs and they did not light up the third on their own. I bought some ebay load resistors but they were so cheap and didn't work.