Why does my amp fuse keep blowing up on my Scion TC?

The speakers are already amplified and unless you bought kit to turn off you cannot put in any amp or radio without blowing fuses and eventually system. They are proprietary as most cars are now. Harness does not turn off amps in speakers but is needed too. Kit is around 75.00 to keep using any existing stereo that came with car which is a pioneer with 150 watts of power or upgrade which is 300 watts. Basically by not using kit you are using 2 amps on each speaker as well as stereo unit.