why is my brake light stay on in the dashboard bmw 325i?

I take it you mean the ABS light.

I have a similar problem.
My centre high brake light blew and blew out the other brake lights. I thought the ABS light was connected to this problem as it all happened together, but seems not.
The ABS is when the wheels travel at different speeds, you apply the brakes and one wheel tries to lock up as you effectively are stopping all wheels at the same rate. When one locks up you could go into a skid.
ABS stops this by sending a signal to the computer (each wheel has a speed detector on it) and it stops brake fluid going to that locking wheel, until all wheels are at the same speed.
You may possibly have a faulty brake sensor (these cost about £16 at BMW or £12 at Eurocarparts).
http://www.eurocarparts.com/ecp/c/BMW_3+Series_316_1.6_1996/p/Car-Parts/Brakes/Brake-Friction/Brake-Pad-Wear-Sensors.
Easy fit! Wheel off plus 10min if you know which wheel it is.
Unfortunately to reset the dash light or find which wheel has the problem, a Diagnostic tool has to be fitted, BMW have quoted me £50 just to put on the diagnostic tool, Quickfit said £30. I tried all my local brake people but the tool is expensive so most don't have one.
This is before any repairs (if you can't do the repair yourself).
Your Brakes will still work! Just your ABS(anti skid) won't. Probably an MOT failure as this shows a brake fault.

Bob
September 2010

check your brake fluid level,if it is down have a look at your brake pads for wear

wickham
July 2010