I don't know much about many cars, but have you removed a stock radio unit to try this? If so, it could be that it was using a non-standard system.
I do however know a bit about electronics, and logic would suggest that there is something wrong with the loom - Your battery would slowly lose charge if not used and then your car would fail to start.
I realise this is obvious stuff that i've just said, but just talking through things can sometimes help. Do you have a car electronics store / good electronics friend nearby that can go through your wiring?
It may also be worthwhile stating your car make/model and the previous unit you removed to allow others to help you out.