I too have a burned chemical smell in the cabin of my 04 Silverado (226000 miles), it began about two months ago. I can hardly stand to drive it. At first I thought the blower was picking up exhaust under the hood, but that is unlikely. Besides the smell is not of exhaust, but rather I imagine vapor from a chemical dripping on the hot engine or exhaust. If anyone has any ideas of what to look for, please help.
valve cover gaskets leaking oil onto a hot exhaust manifold . . . ?!
Any brake work lately? Fluid being squeezed from caliper out reservoir is a possibility. Power steering fluid. Look for trail leading to exhaust. A sweet smell is antifreeze. Check and monitor all fluid levels. If any are low that is suspect.