Panasonic DP801; can't figure it out

I bought a used 1998 Volkswagen Jetta with a Panasonic DP801 8-disc CD changer in the trunk, and can't figure out how to use it. The previous owner didn't know how either.

I put a CD in one of the slots, turned it on with the controller by the seat, and when I push the CD button on the radio, it says "No CD Changer".

I checked in the trunk, and it looks like there's a cable plugged into the side. What else should I check?