Beaver53,
I am not 100% on this, but will say vapors would never make it through the vacuum wiper motor, its little valve, through the chamber vent and into the cab. Under normal operation, this system is under negative pressure, or vacuum.
The only time this could conceivably happen would be when the truck was not running, and that isn't likely to happen.
A penetration through the firewall would allow engine compartment vapors to enter. See if any holes aren't plugged.
Stuart
