Please navigate to GTAIV/common/data and edit the following files for the fbi slot, or rename to a vehicle of your choosing.


Step 1.

carcols.dat
fbi,    	118,123,124, 10,10,10, 0,28,50, 38,3,6, 175,177,177, 38,55,57,


Step 2.

vehicles.ide
fbi,		fbi,		car,		POLICE,		POLICE,		VEH@STD,	VEH@STD_RI1,		100,	999,		0.2345, 0.2345,		0.2,	2,	1.0	,1,	-


Step 3.
Navigate to GTAIV/pc/models/cdimages/vehicles.img
Replace fbi.wft and fbi.wtd with the model files provided.


Step 4.

Enjoy!


Credits:
Original model - Rockstar Games
Edits to unmarked cruiser - aerodynic (PCodeCreatives)