u dont have to use the supplied wiring for your car stereo
all u need is constant 12v and and an earth point, then your stereo will be on 24/7
you could just run a wire direct from your battery through bulkhead as your stereo should be amped already
as for your wiring issues look for black / red / yellow wires and use volt meter with ignition off to find constant 12v, there is one as you will find a lot of wires are on the same circuit as your internal lighting etc
method of elimination
the standard setup was red / yellow one for ignition one for constant live