This mod will create a new reward after completing certain sets of missions. It will add parked Gang Vehicles on all three Police Stations, which will always be unlocked, and will be yours (Cops won't chase you if you enter them). There will be two gang cars in Portland, 3 in Staunton Island and 2 in Shoreside Vale. Here's the list of the new vehicles:

Portland LCPD:

Diablo Stallion - this vehicle will be unlocked after completing all El Burro's phone missions.
Triad Fish Van - this van will be available after destroying Triad Fish Factory.

Staunton LCPD:

Yakuza Stinger - will be unlocked after killing Kenji, in 'Waka-Gashira Wipeout!', and will be parked in near the parked Sentinel, in front of Staunton LCPD's main door.
Yardie Lobo - will be available after completing all of King Courtney's phone missions, and will be parked at the same impound as Yakuza Stinger.
Mafia Sentinel - this car will be parked at the same impound, and will be unlocked after killing Salvatore, in 'Sayonara Salvatore'.

Shoreside LCPD:

Hoods Rumpo - will be unlocked after completing D-Ice's phone missions.
Colombian Cartel - this vehicle will be parked at the same parking lot as Hoods Rumpo, after completing the last mission, 'The Exchange'.

At every Police Impound there is an Info Pickup with introduction to the impound.
These vehicles will be useful because they'll be yours, and you won't have to steal them and be chased and shot by gang members, as well as chased by the cops.
After unlocking every vehicle, you'll receive a pager message saying that a certain vehicle is delivered at certain impound.
But, there'll be more parked cars on the streets as well, like Trashmaster at Portland's Junkyard, more vehicles on parking lots and sidewalks, and FBI Car on Staunton's LCPD staff parking. For now, Black Rumpo is added at Salvatore's place instead of Mafia Sentinel, so now, there is a Black Rumpo and a Mafia Sentinel parked there.

Download comes with an installer, and all you need to know is in it. 
You can find GTAIIIModder's Save Game for this mod here.