Since nobody answered, some general advice (as I don't know GTV 2000 well):
1. Use a switched source. preferably for accessories (Acc).
2. Consult the owners manual which fuses on the fuse panel are switched, then use a multi-tester or test light to confirm which one(s) relate to Acc power. Remove that fuse and test again to see which side is battery and which side is load, then follow the wires from the load side and locate one that goes near the radio. Usually radios have a separate 5x20 mm fuse in the power wire. Also, if the radio picks up noise from the ignition, you may have to install a noise suppressor.
3. The enclosure of the car radio is usually metal that gets grounded by installing a support bracket at the back to eliminate stress on the front fascia and threads for the control buttons. So, finding a good ground is more or less a mechanical problem of finding a good location for weight support.
A scanned copy of the user manual can be found on this site
[SIZE="1"]'63 2600 Touring Spider (AR 191437, the car that started the 2000/2600 International Register, reassembly in progress)
ex-'65 2600 SZ (AR 856043, the car in my avatar, sold as resto project to Austria)
Maintainer of a private 2600 SZ register (not the one in the Netherlands).[/SIZE]