Car Stereos: Charade wiring for car stereo, ground terminal, constant power

Question
We have just purchased a Dihatsu charade and while trying to install a stereo in it I have noticed that the car seems to be wired with non-stand colours.. This isn't a major concern as I have simply tested the wires with a multimeter although I don't undestand why the constant power wire comes up as 12v (all good) but the 2 that appear to be from the accessory switch come up as -12v.
Is this negative reading going to blow up my stereo and / or fixable?
I have kept the ground terminal on the same ground wire that gave me the correct reading with the constant power wire.

Thanks heaps for any help you can give.

Answer
Hi M,

You shouldn't be wiring to the factory wires in the first place.  Always buy an aftermarket harness.  That way, everything is handled for you.

Having -12V on the accessory is incredibly odd, as that would suggest some polarity issues on your ground.

If you are in fact getting -12V on the accessory to ground, and don't just have the multimeter lines reversed, your best bet would be to wire in a relay to give you +12V to accessory.

Your overall best bet, though, would be to put the factory wiring back and buy a harness to ensure everything is wired correctly.

Justin