Car Stereo Volume Goes Up and Down Erratically

I have a 1986 Toyota Truck with Kenwood amp about 16 years old and a Kenwood deck about 7 years old. The volume goes down so low I can't hear it and then gradually goes all the way up until it is blaring.

My question is, would the deck or the amp cause this kind of erratic volume control? This is a beater pickup truck that I don't use alot so I am trying to avoid spending unnecessary money.

thanks