[Diablo de la Noche]

You say the interference gets louder as you turn it up. It may be a power draw. The main power wire you spliced into may be overloaded, maybe due to corrosion or a frayed section of the wire.

Try isolating the entire system. Ground the stereo to the chasis/body, then run all new wires to the speakers, out of the truck, like on the seat. Install the head unit, run it again. Then put one of the speakers in, etc. This should get you close. In an older vehicle you should run all new stereo wires because you never know what was in there before. Old vehicles and new stereos sometimes don't mix well at first, but eventually it will work.