The rear glass may have been replaced on the cheap, omitting the heater grid, or it may not have been available at the time of replacement. Other connectors may be for closure switch, power lock, or lighting (CHMSL, courtesy, license plate). My 2001 has a wire/connector at each end of the rear glass for the heater, which seems silly (should be a single harness and connector, but they didn't ask me). I also replaced the HVAC control head to add the "Rear" option for heated mirrors. Mine wasn't originally equipped with rear defog or heated mirrors. When I replaced the rear glass years later, I added the defog option (the harness connectors were there). The GMT-400 models may be similar, in that the HVAC control can be swapped with or without the "Rear" option. It's possible your control was replaced with the option, never having the rear defog glass option.