Line up 25-30 cars in the multi-storey car park in Newport, on the second highest rooftop level. When the top level runs out of cars, go down one level to spawn more vehicles back on the topmost level. If you go more than one level down, your newly parked cars may disappear. When you have a heap of cars bunched together, "accidentally" lose a grenade between all of the cars.
Hint: Avoid crappy cars like the Manana, Cabbie or Perennial since they don't offer many points when destroyed. Sports cars, and the Patriot, Bobcat and Rumpo all give better amounts. If you drove a car up to the top, it will vanish after grabbing a few cars, and I believe the game is limited to 32 cars maximum before it starts removing them from the screen. You'll know when you're reaching the vehicle limit because cars will stop spawning close to you and you'll have to walk half way across the roof to get to them. Every subsequent explosion increases the score multiplier, so if 4 cars worth 80 points each (dollars) blow up one after another, it adds 80 + 160 + 240 + 320.
Alternatively, just spawn cars using the import/export garage, assuming you have completed the lists (the one in Shoreside Vale could attract police however, especially if pedestrians with an IQ of -1 decide to start walking between exploding cars).