Thanks for all the advice guys. The dealership guys had told me how much the unit was drawing - I forget the figure now - but they said it was a lot.

So.... I decided I needed to remove the lower dash and cowling around the steering wheel to get in there and found out what was what....

I was wrong - they didn't partially remove the unit. I found the whole sha-bang in there - GPS control unit, GPS antennae, handsfree speaker, and something else I've got no idea about. I removed the whole thing intact, even saving the wiring. Two wires were spliced into factory wires (no idea what they were, but one is likely to have been an ignition wire - the unit can remotely kill the car).

I've left it a few days and the battery starts the car no problems, so trust the situation is sorted.

And I have a AUS$2000 GPS unit (admittedly second hand and possibly faulty!) sitting in my garage.

Justin