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There is a switch, but the car always starts on petrol even if the switch is in the gas position, apparently they won't start on gas for some reason.

You can switch between fuels whilst driving.
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I think, but stand to be corrected, that a turbo car needs multi point injection (which is the standard now anyway) but an older kit may be single point, which is what I've got on my 850.

I think you'll find that the cd changer carries data to the head unit and not left and right channels, you might be better off opening the head unit and connecting to the cassette heads.
But I stand...