This is the safety interlock switch
Motorcraft SW5163 (possibly also
Standard SLS66) on the driver's latch that prevents the motor from operating with the tailgate open. Ford put it here from ~'85 until '96. Before that, it was on the center mechanism and was sensitive to EITHER latch being open.
To add a simple self-diagnostic capability, wire a 12V lamp across the safety switch terminals, and mount it in the tailgate shell above the lock cylinder in the inside sill (through a carpet retainer hole if present). If either control switch is activated, and the only thing preventing the glass from moving is the safety switch, the lamp will light and be visible to a person using either switch.
To permanently defeat it (and risk breaking the t/g glass), unplug its connector & install a jumper wire between the 2 terminals.
