Yep, in Dead Skunk in the Trunk, that's a Forelli brother sitting in the parking lot. So instead of blocking him in, park a nice big car right next to his and BLOW IT UP! You can't blow up the Forelli vehicle directly (i.e. with your own weapons), but the explosion from another car will carry over, and voila, one less Forelli to outrun.
Better yet, let the Forellis whack you from behind until your trunk falls off. The corpse will be visible.
(Pity that it's a Manana, or I'd keep it.)
Mafia Sentinels are actually among the easiest cars to get, if you know how (or, more properly, where). Go to the top of the hill in Saint Mark's, and take the offroad path leading to a large mansion. There are two Mafia Sentinels (free of charge and unlocked) in front of this building at all times. That should give you a hint as to whose mansion that is, but the big Don doesn't believe in car alarms... They're reasonable all-around vehicles, but there are certainly better ones.
The Fire Truck is in the north end of Portland -- Harwood, I think. It's not too far from the auto dealership (where you can usually steal a Banshee, which is lightning-quick) and across from the gas station. Grab it, turn on the vehicle missions, and you'll get to drive recklessly around Portland putting out car fires. The type of car that you extinguish is randomly generated -- you can get good stuff like Cheetahs, Stingers, Yakuza Stingers, Infernuses, etc., or oddball vehicles like the Mr. Whoopee ice cream van or the Trashmaster. Again, the Stingers are highly recommended for Turismo, as they handle and powerslide like a dream and have a good top speed, though it may take a lot of missions to find one.
Oh yeah -- complete 100 taxi missions (not in a row) for a pleasant surprise...