Extra amp with mach 460

Doesn't sould like you have much of a choice (besides getting a new HU).

not sure of how ur head unit was installed...usually with an after market head unit u have to have the soche adapter to adjust the high level outputs to the to lower line level. This will eliminate the pop and hiss.. u do not even use the rca fromt he head unit. this would would leave the one head unit rca free for the sub....

other option is to go by a radio shop and have them splice into the wires leading to the mach 460 rear speakers... I had mine done at best buy...it was less than 50 for the line level conveter to rca and install...i did this when i was still using the factory head unit...

i later added an after market head unit and did as described in the first paragraph...