Not sure exactley where on your truck the switch will be located but places to look if it still even has one(if it hasnt been removed by someone and converted to straight lpg only)....is there a switch on the dash??...other places are to the rear of the truck close to the radiator support or on the sides...as for the no start...because you smell propane i would check to see if it is getting to much propane PSI..sounds like the vaporizor may need rebuitl..if it is the impco vaporizor the fuel psi should be around 2-3 PSI coming out of the vaporizor and to the carb(air/fuel mixer).....