ford ranger electrical system malfunction

Well I have installed a lot of radio in fords easy swaps with dash kits and wire harness, but this one time there was already an aftermarket radio in there the customer said he wanted a new wire harness after I wired up the new harness 2 the new radio I tested the radio and nothing and I mean nothing so I check everthing I even conected the radio 2 a power supply and it worked so I looked at the old radio and harness and noticed the batt and acc wires wher connected together so I tried it and boom it worked I was puzzled becuse the wire harness said nothing about that. Oh and it was a late 90's ford f150 ext.cabwell?