What kind of slide is it?
For the lci swinteck slides there are controllers for them. The controller is labelled on the 12volt input. The output side is what would be reversed. They are simple push on spade connectors. Swap the output wires and that should fix the problem. On our trailer the swinteck controllers are mounted in the basement on drivers side on the floor joists above your head. I found removing the mounting screws and let dangle on the wires easier to read and manipulate
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