Eddie, sorry if I've confused. When I switched the wires the LED came on with the running lights but doesn't get any brighter when the brake is applied.
Are you expecting the light "to get brighter"? Because that is not how a tail/stop lamp works. You should have two positive cables (and one ground). The first will be connected to the side light feed and will drive the standard brightness LEDs, as a tail lamp. The second will be connected to the brake light feed and will turn on the higher brightness LEDs as a brake light.
I used a Landrover replacement dual brightness LED tail/stop lamp unit which is a straight swap for the Cobra high level brake light. As I only needed the bright brake light, I didn't connect the standard brightness cable. But it was easy enough to check the function of the two separate circuits to determine which was which.
Nigel
