i tried to put in subs in my 2000 bmw 323i but the installation guy said tht i need to change factory stereo to aftermarket because theres some sensor in factory stereo and has tht flashing red light and if stereo is taken out it does something to car so i would like to put subs in but w/o changing factory stereo

You can tap into the rear speaker wires using a LOC (line output converter) to convert the speaker signal to RCA.
There is no need to change the radio.
run power and remote wires, ground the amp, hook up the RCAs and the speakers and you are ready to go.

ok what the guy told you is right kind of your stereo has a anti theft system so if its disconnected you need the code to turn it back on otherwise it wont turn back on so what you need is to tap into the rear speaker wires using a LOC (line output converter) to convert the speaker signal to RCA. you dont even need to touch the stereo just connect the unit to the rear speakers, the amp is installe normal and the remote you connect it to the unit and your all set as for subs you can put 1 or 2 what ever you want subs arent made specifically for a certain car so good luck