It might just be better to build a gain control into your amp-either way you're gonna need 2b handy with a soldering iron and stripping coax, as well as careful to keep your exposed signal leads shielded as best as possible (usually less than 1/4 inch exposed length) to reduce EMI noise risk. Still wanna do it? ok-for starters, you'll need a dual 10k ohm to 50k ohm pot(if your old amp is like most and has a 500mv input sensitivity, go with a 20k ohm pot if u can find)...use the circuit board type if you're gonna drill a hole in your amp case and mount it to show like a newfangled amp, or go with a larger, volume-control size dual pot if u insist on having it up front. There is the cheaper single turn type, and there's the 10 or 20 turn precision type that I much prefer, although they are about $20-$30.00. From here, just tap into the rca cables somewhere , soldering one side of the signal wire (say head unit side) to the leftmost contact if th pot handl is facing u and th contacts r up; connect th amp side to th middl contact-leave th right one open, I don't like to shunt any signal to ground, no-no. Connect both sides of th shielded part together. Repeat for other channel, mount pot,and u have take-off. Or, u could just buy a new amp...