Diagnosing drill with no reverse

I have a Porter Cable hammer drill (#7750) and the reverse isn't working. I have opened up the handle and accessed the trigger switch, but there are a few too many wires going into it for me to figure out where to test. I see the main current wires(one white and one black) coming into it, and the two wires(one black and one red) going to the two brushes. But I don't understand how the reversing action occurs. Is it a simple reversing of the polarity going to the brushes? If so, how to test? Do I have to test it live, or energize the wires with a small battery? I'd like to ask for some ideas on this.
John
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