Eigen, you may be thinking of a much more recent staged "test" of a pick-up truck exploding after a side impact, done by the competing manufacturer to make the other's product look bad.
The Pinto had two issues with the fuel tank in rear-end collisions; the first was that the back of the rear axle had very sharp edges, and the tank could get gashed open if shoved into it. The recall fix was to install a plastic cushion between the axle and the tank. The other issue was that the filler tube was simply inserted through an O-ring sealed opening in the tank a small distance past the seal, and it took only a slight movement of the tank to pop the tube out, leaving a 2" hole. The recall fix was to install a longer tube, that didn't pop out so easily. Ford knew about the problems, and decided it was cheaper to pay off the claims than fix the cars, until that landmark case.
Recent Ford Crown Victorias, often used as police cars, have also earned a reputation for bursting into flames after rear end collisions.