Years ago I tried placing the heat ducts in different location the results were night and day. Forget about routing the heat through the 2x4 steel box frame the heated air passes through the metal mass and by the time it exits at the heater door slides it's almost cool. Troy has a video showing the amount of heat that exits direct flow via PVC pipe and that method seems to work. I tried two methods: Direct from the heater box to the carpeted area L & R door jamb it worked great but it was noisy. Then I tried direct from the heater box to the carpeted vertical area of the rear seat, that was the fix ...it forces hot air around and under the seat . The negative: both of these methods does not allow for the heat to reach the windshield .