but what on earth does this mean?

No pun intended, I'm sure.

It means there's something really really funny going on with the ground/audio wiring either in the player, in the sled harness, the amplifier, or the wiring that connects it all to the car power.

For all intents and purposes, the sled body and the ground wire coming off of the sled harness should be at the same potential. What you described should not happen.

Obviously you can't live with the cardboard on your player. You need to be able to slide the thing in and out without messing with bits of cardboard. This shouldn't be happening at all, there's something else wrong that needs to be fixed.

Have you opened your player and looked for loose/frayed wires? Perhaps if you contacted David (and pointed him to these three threads) he would give you permission to do so.