I also have a Pheonix gold Amp for my sub. If the knob is a Potentiomenter you can get the info off of the Pheonix Gold web site (if they still have it posted) to solder it to a telephone wire/jack. It's pretty easy. Can't remember the details myself though. It's been about 2-3 years since I did so myself with the telephone cord and potentiometer connection.
Terry
yea i'm pretty sure it goes to the back area, there are 2 wires under my back seat that probably go to the knob in the console. the car was prewired for an amp when i got it, but with shitty rcas, 8G wire and a horrid ground, so i doubt it was a quality amp. i have a pheonix gold amp back there now and it has a cub control input, but it's a phone cord jack so i dont know how i'd wire this thing to my amp.