The heat in my 97 Silverado, smells like melting crayons, and gives me a headache. Have not noticed the smell with the A/C, so presumed it was a heat related issue (as opposed to odor coming from the engine compartment or mold on A/C condenser.) Took the dash, etc. out and got all the way to the heater core (man what a job!) expecting to find something melted on the core, or a leak. Nothing on the core, no obvious leak (very minor corrosion on one tube & fins but no apparent leak.) I pressure tested the system to see if the core would leak (can't run it with all the electronics out,) but no leak. Had been losing coolant, was not obvious why. With the pressure tester pumped to 16 psi, the engine oil cooler fitting into lower left radiator fitting started to leak radiator fluid, as did the upper hose at the thermostat, and discovered the outer gasket for the radiator cap has disintegrated (replaced radiator.) Obviously, fix the known leaks. Cleaned all of the duct work from the dash, cleaned as much of the heater box as I could reach my hand into (can't take the whole thing out because A/C condenser is inside, another brilliant design,) no change to the odor. Also noticed that when plastic parts were left out in the sun and got hot, they gave off the same odor as when heat is on, has anyone else ever heard of such a problem with the 97 trucks?