The firetruck IS a good way, but I think I've found a better way.
It involves two of the hidden cars as they are worth MUCH more than the firetruck (According to the social club) and, more importantly, the are MUCH quicker to destroy.
The "Phantom Corsair" can be found towards the East edge of the Map, just West of the River at the corner of Traction and 3rd, in the petrol station.
The "Duesenberg Walker Coupe" can be found just off Santa Monica Boulevard. Head East slightly from the corner of Santa Monica and Vine and go through the archway into a carpark.
Both hidden vehicles are found by going into the blue garages.
I did the case "The Drivers Seat" and basically smashed up one of each car (until its on fire, by doing head on collisions only at full speed) between each case section. This provides an autosave after every couple of cars in case you screw up and die (be sure to jump out of the car after each collision, if its not on fire jump back in and crash again).
These cars are worth over $20000 each, according to the http://socialclub.rockstargames.com/
so even if its not completely destroyed you are going to get a hefty bill.
Some TA'ers have posted that they are getting $7k-10k per car rather than its full value. This means you cant just write off 3 or 4 of them and have enough but doing one of each car between case saves as described below should still earn you enough. Each pair of cars should give $14k-$20k using the values other members have found. This means 4 pairs of cars should
be enough. Always worth going for more though.
Fast travel to each cars location, the money you'd get smashing things up along the way is negligible. I found, generally, it took 3 full speed head on collisions to destroy each car.
By simply following
a) smash up both cars
b) do next case section until it saves
and repeating until the case was solved I easily got the damage needed (i finished on ~$85000)
The case is quite a short one as you only need a few clues (receipt and drivers licence at railtrack, matches at house), you only need to interview Morgan (lie-receipt, doubt).
Hope it helps
EDIT: Thanks for the comments guys, especially those with total damage and amount of cars to help work out a more realistic value for each car
EDIT 2: A couple of people have suggested keeping track of damage here... http://socialclub.rockstargames.com/games/lan/casetracker
So, I've added it to the solution. Thanks to NateEasy for the tip