Weird electrical problem

OK.... I'm having an issue with the wifes jeep('00 WJ). The problem was pointed out to me by a "helpful civil servant" last night. The brake lights are malfunctioning in that they work fine until the driving lights are turned on, then the brake (left and right) completely stop working, and the third light stays on with or without depressing the brake pedal. I have changed all the bulbs and fuses, problem still there. Any ideas???

It may be the multi-functional switch. Did you check the bulbs for being the right kind? You can't have a single element bulb in where it should be a double.

I'll double check the bulbs. That doesn't explain why the third light stays on...

Unless the third light is activated by the taillights using a single element bulb in the tail/brake light sockets.

sort of like a 3-way switch concept? that makes sence.