My brothers had these in the early eighties. There were two plastic cars (I think one red and one blue) and each had a toothed plastic strap that was pulled through the car roof. This set the cars off and the aim was to smash them into things or each other.
Various panels on the cars then fell off - I guess the winner was the one with the most panels remaining.
Do You Remember Stock Car Smash Up?
Do You Remember Stock Car Smash Up?