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If there is a black tracer line on the speaker wire that is more than likely negative. If there isnt a tracer, get a 9 volt battery and hook it up to the speaker. When the speaker pushes out the positive and negative are hooked up rite, so jst remember what wire hooked to what connection on the speaker.
Get a digital multimeter, set it to dc, turn the radio on (make sure the wires arent touching each other or anything else) and hook up the leads. If you have them wrong the - will show.