This is by Ford in 96 Owner's Guide;
Guarding Against Exhaust Fumes Carbon monoxide, although colorless and odorless, is present in exhaust fumes. Take precautions to avoid its dangerous effects.
"...Make sure your Bronco’s tailgate window is closed when your truck is running to prevent exhaust fumes from being drawn in. If you must have the tailgate window open, adjust your air control system to force outside air into the front of your truck. If your Bronco has outside air control vents, open them fully. Have the exhaust and body ventilation systems checked whenever:
your vehicle is raised for service
the sound of the exhaust system changes
your vehicle has been damaged in a collision
Improve your ventilation by keeping all air inlet vents clear of snow, leaves, and other debris.
If the engine is idling while you are stopped in an open area for long periods of time, open the windows at least one inch (2.5 cm). Also, adjust the heating or air conditioning to bring in outside air.
HEATING — Set fan speed at MEDIUM or HIGH, the function selector knob on VENT,
FLOOR, MIX, or the DEFROST symbol and the temperature control knob on any desired position.
AIR CONDITIONING — Set the fan speed at MEDIUM or HIGH, the function selector knob on NORM or VENT and the temperature control knob on any desired position..."
rusted out floor pan
floor pan plugs missing; Many plugs and grommets are used in the floorpan and dash panel. The floorpan plugs seal the various access holes. If any plugs are missing or improperly installed, a dust or water leak may result. This also applies to the grommets used on the dash panel. When dust or water leaks are evident, these plugs and grommets should be checked for proper installation.
quarter panel wheelhouse inner panel rusted out into cargo area; seat & seat belt belt bolts missing with holes in pan;
Underbody Heat Deflectors bolt missing
Body Sealant deteriorated @;
quarter wheelhouse inner panel and quarter wheelhouse outer panel
roof side drip rail
Check weatherstrip @ floor pan where bottom of tailgate meets it; air, fumes and water can be sucked-in through a bad ws.
Liquid Butyl Sealer
Liquid Butyl Sealer C9AZ-19554-B (Black) or equivalent meeting Ford specification ESB-M4G162-A does not run, is fast drying, and remains semi-elastic. The sealer can be used for seam sealing in such areas as the floorpan, wheelhouse, dash panel, running board, door openings and drip rails. It can also be used to seal outside moulding clip holes, and for windshield and rear window installation.
Caulking Cord D6AZ-19560-A or equivalent meeting Ford specification ESB-M4G32-A has a plastic base with a filler, is heavy bodied, and is commonly known as perma-gum. It is used on spot-weld holes, around mounting clips and between two surfaces not sealed by a gasket.
Weatherstrip Adhesive E8AZ-19552-A or equivalent meeting Ford specification ESB-M2G14-A is a quick-drying, strong adhesive designed to hold weatherstripping to doors, bodies, cowl ventilators and the surrounding metal. Windows and windshields that are set in rubber can be effectively sealed against leakage by flowing adhesive into affected areas.
Clean all grease, dirt and old sealer from surfaces, and wash the surfaces thoroughly with a cloth moistened with a suitable commercial cleaner. For best results, apply a medium coat of adhesive to both surfaces, allow it to dry until tacky and then press both surfaces firmly together.
Silicone Lubricant C0AZ-19553-AA (jelly) or equivalent meeting Ford specification ESR-M13P4-A is to be used on the door and window weatherstrips. It is recommended that silicone lubricant be applied to the weatherstrips at every lubrication period. Its use makes the doors easier to close, avoids weatherstrip squeaks, retards weatherstrip wear due to chafing between the door glass upper frame and the weatherstrip, and helps to retain door window alignment by reducing friction between the glass frame and rubber weatherstrip.
Dust and Water Leaks
After trim removal has revealed the location of leaks, seal these leaks, and road test the vehicle on a dusty road to make sure all leaks are sealed. Dust entry is usually indicated by a pointed shaft of dust or silt at the point of entrance. After the road test, check for indications of a dust pattern around the door openings, cowl panel, and cowl side panel.
Sometimes leaks can be located by putting bright lights under the vehicle, with the above components removed, and checking the interior of the body at joints and weld lines. The light will show through where leaks exist