can a add a subwoofer to my stock radio in my 2003 mazda protege5

No you cannot. Factory radios do not have the extra RCA cabls and wiring you need to add a subwoofer. You can pick up a new radio for around 40 bucks on eBay with many more features than your stock radio. You need alot of supplies other than just a radio and a subwoofer. If you still need help just reply.

YES you actually can u need a special adapter i think its called a bass controller, it makes the sub woofer louder when you turn up the bass instead of the woofer and the speakers. they use this method alot with newer cars that have multiple controls in the stereo head unit. i had to use this method in my VW so it did not void my warranty