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No adjustment just for REV lights. Shift cable may be slightly out of adjustment.
With engine off, press and release the button on the shifter handle while cycling through the gears. Determine if the shifter detent engages in the same position as the tranny detent.
Once the tranny is in a gear, while releasing the button, if the shifter tries to move forward/backward, then the shift cable needs adjusting.
If this checks good, then might have a bad Neutral Safety Switch on the tranny.