Managed to do some very quick testing of the power supply to my dvd rom. There is a constant 12v which I presume is to preserve data in the flash rom, however hoving to position one on the ignition switch sees this drop to 11v, go to position two and it's hovering around 10v, didn't get to cranking but I suspect it'll drop under 10v. Going through the ignition switch avoids this to an extent. I took these readings from the dvd rom power in so I can't 100% rule out a problem with the dvd rom but it's unlikely. Should there be a voltage drop like that? It's a new ignition switch by the way. - - - Updated - - - Just thinking this out, a bad earth could cause this. I wonder if the earth from battery terminal to chassis is sound? Where are the other grounds to check?