Car stereo not working?

I bought a used car and the connector peice that goes from the car to the stereo has been cut off and its just wires now. I tried rewiring a new connector peice that i pulled off in a junkyard and there was no main power wire in the new connector to wire so i just bypassed it and i wired the two straight together and it didnt work...I've wired head units before but they never had the connector peice missing. What should I do? or who can i take it to so they can fix it?