Maybe it "IS burning" plastic, -- or at least plastic that is real hot! Check under hood immediately after engine shut off, and check to see if anything hitting on exhaust mainfolds, or maybe engine block, - or water hoses. inside car see if smell coming from heating/cooling "registers" in passenger compartment, a heater motor going bad occasionally causes the "voltage dropping" resistor to overheat when running on slower speeds, - which in turn will transfer to "plenum" housing when fan not "moving air"! I have also seen "melting plastic occour with courtesy lights under dash, if the bulb is hitting on side of plastic mount/housing (when doors are open)! In short follow your nose - you will probably find it after a while by narrowing down what "isn't causing smell"!