Usually, Red goes to + voltage (12V or battery) and Black to - , ground, chassis. If you've done that, and it doesn't turn on, you may need to add a line from +12V to the Ignition Sense terminal; depends on whether you have a single-piece dashmount radio, or a remote mount, which has a control head, and a large cable to a package in the trunk.
Need more info on your radio;