"heat vents"? you must mean the fresh air vents on the kick panels, no heat involved
behind them the cowl-top grill feeds into plenum chambers on each side, and that airway carries on below the kick panel vents to a drain at the bottom of the cab right where the lower rear fender bolt is - better start taking a closer look at how the cab is built, there's alway a lot of rust in those plenums, from the top rear fender mount point right to the bottom drain, and usually involving the lower hinge pocket as well .... probably pics in the assembly manual that show the area
Bill
