Hi Don,
Not too long after Petty retired, I saw models of his Grand Prix with the nose tore off, as it appeared in Atlanta after his last race.
Heat is one of the best ways to produce "dents", but you can't get to excited because it will start looking more like melted plastic! To make torn sections of sheet metal, remove a section of the model to be affected and insert a jagged piece from a drink can, using putty to fill the gaps between plastic and tin...... Use a photo of a real wrecked car as your guide. (extreme weathering using soda can tin can be hazardous to the capillaries and veins in your fingers and hands) *obligatory precautionary statement*
"When did it start making a noise like that?"
Frank