TIPS: Probably very few people play DPL these days. However, I have a few tips on the rarest cars for those who still play this great game. This forums helped me a lot, so I feel like I owe you some feedback.
First of all, if you can’t find some cars just complete more missions and some new vehicles should spawn in the streets.
Melizzano – probably the rarest car in the game. In fact it’s very easy to find. The rule is simple. You won’t find this car if it doesn’t show up on the streets. So, the only way is to start the mission in which Melizzano spawns. Of course, it only appears in ’78. The best mission to find it is the Street Race – Hard in New Jersey. BUT!!!! And this one is very important. In case you’ve forgotten about that. There are three Hard Street Races in the same point. So you have to complete the first hard street race and then just go to your garage, repair your vehicle and prepare for the second race. You may find yourself confused by the fact that it looks like exactly the same mission. But it’s not. If you approach the same red dot (a mark on the map) the second race will begin. And now you should notice that one of your competitors is driving Melizzano. I assume that you may want to win this race (without failing) and get mezzaliano as well. And so, you’ll notice some Melizzanos in the traffic during the race, but if you leave your vehicle you’ll automatically loose this race. So instead of that just go for your victory and when you reach the finish-line just wait for your competitors. No you can’t steal your competitor’s melizzano after the race. But as long as it’s still there, even though it’s closed, you can find some melizzanos in the streets. Just leave your vehicle and go on foot down the street. I went up the north. It may take a few minutes but don’t give up. I found mine at the crossroad.
San Marino and Chauffeur – It’s very easy to find them by approaching Steal to Order – Hard. Of course, you can get those cars without failing the mission. If the destination mark appears near Colony Island you’ll be asked for stealing San Marino. However, if the red dot appears in Manhattan you should expect to be asked for stealing either Chauffeur or Raven. If it’s raven just restart the mission. When you get your car with a Wrecker just bring it to the garage and leave at the spot. But don’t go for your payment yet. Just look for the car you’re looking for in the streets. As long as the wrecker with the stolen car are at the spot you’re very likely to find the car parked or driving nearby. When you find it, you can go for payment. Mission completed and a new vehicle collected. Well done.
OK, it’s time for ‘06 sports cars. I guess, you won’t be surprised if I tell you that you can get all of them in the same way as I mentioned above. Just approach the same mission in 2006 and you’ll find Zenda, Teramo and probably MX2000 and BX-9. In case, you couldn't find the last two just be patient you’ll find them later during some missions or driving on the streets. And now some Commercials.
Dozer - this one is fairly easy to find at the construction site in Manhattan. However I don’t remember the exact location. There is one at the construction site in Bronx. But I've noticed that the one in Bronx appears only at the very beginning. If you complete a few missions it disappears. So my advice is that it’s best to store Dozer in your garage right when you find one.
Firetruck and Prison bus – funny thing, they spawn on the street at the beginning when you are in the early part of the story. Later on they disappear. So try to store them as soon as possible. Firetruck spawns randomly. If you want to get a prison bus I suggest to visit Coney Island or to drive under the railroad trucks in Manhattan it often spawns there.
If you struggle to find other commercials, you should visit Coney Island and drive over there for about 30 minutes and they ought to appear.
Swat Van – I found it hard to find this one. Don’t waste your time looking for this vehicle if you haven’t completed the whole story yet. I noticed that you’re more likely to find it when the story is over. When it’s done go to the bridge leading from Bronx to New Jersey. Start your game at Hunts point and go there but don’t enter the bridge. Try to piss off some cops in the meantime. Stop your vehicle at the last crossroad and start to kill some cops. You may have to try it a few times.
Security Van – I found it during the mission Gate Crasher. The first of the three locations you should visit is the one in New Jersey. Drive to the tunnel in the south. When you get towards the entrance a Security Van should spawn in the tunnel just leaving it.
And that’s all. It works on PC, I hope you’ll find it useful playing other platforms, too.