Have had this issue with my brake lights on my 99 328Ci for about a year now and it's driving me mad!
When I switch on the ignition on random occasions I get both rear bulb warning lights come up on my dash and the stereo doesn't come on and the button on the end of the left steering wheel stick which lets you switch the display between miles / temperature / mpg etc etc doesnt work and my brake lights come on permanantly. Then after a few minutes the dash warning light / symbol disappears, everything goes back to normal and then the stereo etc. comes on and everything works again.
Had my mechanic run a diagnostics test on it when it does it and it comes up with a brake test sensor error so we tried replacing the brake light switch and it made no difference.
Anybody experienced this before?