2005 Chevy Cobalt LT with factory upgraded Pioneer sound system. No upgrades or mods of any kind. No OnStar or SatNav. I assume something is shorted/broken because the amp fuse is immediately blowing. The fuse is located in slot 6 of the fuse block in the passenger foot well. After the amp fuse blows, the stereo and all other electronics continue to work, but no sound is produced by any speakers. (I say the stereo "works" since it can turn on, change stations, read a CD, etc. but no sound is produced.) To try isolating the issue, I disconnected negative battery terminal, removed the stock Pioneer amplifier, replaced fuse, reconnected battery; fuse still blows. Removed head unit, repeated test, same result. Disconnected all speakers (including sub), repeated test, same result. Also tried different 20A fuses from different stores/manufacturers to make sure didn't get a bad batch. So, the head unit and amplifier are out of the car and all speakers are disconnected. With negative battery terminal disconnected, replace blown fuse, reconnect battery, fuse blows. * What is my next troubleshooting step? * What other devices use the 20A "amp" circuit? * Is there a way to test the fuse block itself? Thank you!