Figuring that out is as easy as simply hooking up the wires in both options and seeing if it works. If neither worked then here are the other potential issues...
- there's a severed or loose wire somewhere
- at least one of the wires may be disconnected or making poor contact at the radio wiring harness, or amp if one is used.
- the head unit is faulty, which is not uncommon if you still have the factory radio.
State if you have the factory sound system or an aftermarket one, and what was installed in regards to radio, speakers, amp, etc.