I would first take a ohmmeter and measure the resistance between the loose wire you discovered, and the yellow wire on the controller. You're looking for near zero ohms resistance between the two, of course.
Assuming that the yellow wire is supposed to be power (12 VDC) then it certainly could explain your slide controller issue!
At any rate, since the crimp on that terminal was bad and the wire came loose, obviously you will want to reconnect it properly.
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