This is EXACTLY the cause. It turns out the yellow IGN wire is just a “wake up” signal to the radio. It takes a minimal amount of voltage to power up the radio, and a minimal amount of power to keep it on.
It turns out the stray voltage was a result of another modification I had made to his truck. I had previously installed a cheap flashing LED security lights in the dash. I had wired it with the POSITIVE on a orange battery power feed (like the dome light, or on his truck I used the power door lock power feed) and the negative wire of the LED to a pink IGN wire (in his case, the wire I used was on the same circuit as the radio) that way whenever the ignition was off, the LED would flash like a security system. Turn the ignition on, the light would go out. It worked flawlessly for over a year, until we installed the retro sound stereo.
It turns out just enough voltage leaks into the radio circuit from the flashing LED to keep the radio powered, which in turn kept the amplifiers powered up, the end result was a dead battery. .
Retro sound suggested using a relay to to power the radio. They wanted us to use the ignition circuit to power a relay, which in turn would power the radio.
We just removed the flashing LED, problem solved. However, after bitching to them multiple times and always getting the same “we have never seen this concern!” reply, judging from the people commenting on this it’s not an isolated issue!