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On the Slacker thing, you can do it through the normal web interface. Log in as the car user (or yourself if using your own account), view the channel you made the mistake on. Somewhere near the bottom of that page there's a list of all the dislikes/blocks. You can undo them from there. AFAIK you can't undo it from in the car UI. I don't think you can do it from the phone apps either.
EXCELLENT! In the Track list on each channel it shows any song that was rated (up or down) -- click the rating to unrate. Thanks.