Volume fades to zero

I have a very strange problem. I have a Pioneer KEH-P4200RDS stereo installed in my Toyota. It has a 12 CD changer in the trunk. It also used to have a Pioneer GM-X552 amplifier and a subwoofer in the trunk, but those got stolen one night. They hacked through the signal, power and remote wires. I have since disconnected the power wire from the battery, the RCA connectors from the back of the stereo and the blue control wire at the inline fuse in an effort to solve my problem. Now the problem --- Everything works fine as long as the key is set to accessory. As soon as I start the engine, the volume fades out. When I put my foot on the brake It fades back in a bit, and a bit more when I put it in gear. As soon as I accelerate The sound fades back to nothing. If I stop at a red light it fades back in somewhat, Then out again when I start to move again. Ishould mention that during cold weather (ex. -20C) everything seem to work almost normal for several minutes until the vehicle begins to warm up, then slowly back to the fade out. CD, Cassette and AM/FM all work normally, so the only thing is the volume fading to zero! I have pulled the deck three times to check for correct wiring and/or bare wires but it all looks good to me. After almost 1.5 years this is beginning to really, really irritate me. Lately I have been thinking that it may have something to do with the Blue control wire? If you can help, it would be much appreciated and could possibly save my sanity! THANK YOU