1999 BMW 328 indicator brake light came on

The only way to tell which brakes require service is to have them inspected. The dealer will usually be more expensive than an independent repair facility, I would get several quotes and talk to the service advisers, they are usually aware that people shop the price around. Just to give you an idea of what to expect, the parts usually cost about 150-200 and the labor to replace front OR rear brakes is about 1.5 hours times the repair facilities labor rate, so with that knowledge you can be an informed consumer. Be prepared for extras such as, flushing the brake system, brake sensors, or other additional add-ons that repair shops might try to get you to buy. If there is anything else, dont be afraid to repost, I am glad to answer your questions, good luck with your BMW.