We used to take aluminum foil and mold it over the bodies, cut out the wheel wells and use a marker to color in the windows and put numbers on. Gently peel them off and use a thin strip of scotch tape to run under the chassis to hold them on. You had to be really careful driving because when a wreck occurred it did real damage to the body like a real car. Crunched front ends, fenders, and quarter panels and if it flipped the roof was crushed. A little touch of realism.
The Hot Wheels jumps and lighter fluid is a story for another time...
