Brake light problem

I have a 91 BMW 318is and the brake lights are always on. I checked the switch under the pedal and its not stuck and seems to be functioning. I need to know what the problem might be?? and if I can fix it myself as I really do not have much money to take it to the dealer or a shop.
Thanks for any help

could be internal on the switch.. try to unplug it.

I have seem this happen when the wrong bulbs are installed

does it have the brake pad sensor??