althought its not a 3rd gen...my bros neon has my old 200M fostgate amp powering my old 12'' Mtx 6000. it worked fine until a couple days ago...hes using the speaker channels for the inputs instead of rca's. and has it connected through a 2nd fog light switch (looks stock) and as soon as he turns onthe sub theres a continous rumble...as soon as he turns it off it shuts off....my first guess would be a ground somewhere between the switch and amp. but beyond that i would be clueless....so if anyone knows or has experienced a constant rumble (besides our engines) and might have an idea on what it could be...any help would be greatly appreciated.

 

Try disconnecting the speaker inputs. How is the amp grounded?