Ive gone for the different approach. I pulled the roof lining down by my aerial stump and took the feed off the connector in the roof. Its easier becuase its at the back of the car where you want the amp to be located. The aerial only activates the remote signal when the radio is switched on, and so the amp only switches on when required.
However, I fitted sub/amp to standard stereo of the birds mini cooper s and it seems the aerial is always on when the engine is running, but when engine is off, the aerial is off until you manually switch on the radio. So i know it works, just that its permantly on whilst car is running. Any suggestions on how to overide this and only have amp go on when radio is on????