I'm confused on when it comes to the "20 dollar harness" I understand its purpose however I couldn't get the part number to come up on metras website and according to crutchfield no harnesses fit my 2004 grand prix with onstar and monsoon except the really expensive one that retained everything but there were alot of negative reviews on it and I can't really afford it right now at least. So my question is if I read the manual and match the wires on the power harness coming with the HU with a list from the12volt.com on which wire is which inside the car that should be just as effective as the "20 dollar harness" right?