Acura TL - nothing to use for Amp remote

Hey guys, got one for a pro to toy with: I've got an '05 Acura TL w/ Nav. Installed an amp and sub using a PAC RCA converter w/ built in remote output for the amp's signal inputs. To acquaint you with the car, it has 5.1 DVD-A surround sound w/ a front center and rear sub (each w/ their own volume controls). I used the rear subs speaker wires for the signal to the PAC RCA converter so I can control the aftermarket sub volume through the OEM radio. Sub sounds great but ran into a problem; when I turn off the radio I'm getting that feedback "reverb" problem associated w/ an amp that's left on w/ no input signal. The problem is the speakers continue to have a signal w/ the radio off so the Navigation voice can still work, so the PAC converter never cuts out the remote signal to the amp. I've searched the car over w/ a volt meter both at the OEM amp and powered antenna and can't find a 12v signal to use as a remote that turns off w/ the radio. It seems now the only options I have are 1) find another amp that doesn't give feedback when left on w/ radio off, 2) install a toggle switch to manually cut off amp (really don't want to have to do this), or 3) have no sub or amp (not really an option at this point). Any one have any suggestions??? :confused: