If you have a proper accessory feed at the head unit then this is the best place to pick it up (B6 vehicles don't have it as they use the CANBUS, but your cabriolet maybe different). Pull the HU and you will have a black power ISO connector. It will have a brown earth, red with white stripe permanent live, and hopefully 3 pins along from the earth you will have a yellow with red stripe wire, this is the accessory feed (ignition on which stays live until you pull the keys out). You can use this to trigger a relay that then feeds the PS2. I wouldn't wire the PS2 directly to it. It needs a relay.