I'm not sure about the amplifier - I couldn't see anything that looks like it plugs into a standard ISO radio connector... There is a 2 pin connector dangling behind the glove box with nowhere to plug in, but it looks like a VW plug, not an ISO (or bullet) plug. Without knowing what this is I'm reluctant to shove 12v up it! Know where the amp is supposed to be? It's not by the glove box, deffo...
As for earthing the middle wire - on my mk2 I insulated the shield from the connector (so the pin was the only thing making connection) and that worked ok but I'm sure things should work without resorting to bodging it!!
And BTW I do have a beesting....