I found a small tip which works pretty well for me, let me explain
As some of you did, i finished 90% of the game, including side missions and i totally ignored the Unmarked Police cruisers in Barry's mission. As I really wanted one, i did the underpass loop but i only found civilians car and sometimes a patrol car passing by.
Actually, in order to find the rare cop vehicles which spawn under this bridge (Police Van, motorcycle, unmarked etc ..), just take a police car and do as if you were patrolling the district, always do the same path, start from under the bridge then go away a little far in order to reset the vehicle spawn and come back. I did this technique a few times to get the unmarked crown vic, although it's easy to get, it's unfortunately always black.
Another way to get these, in a random color this time, is possible with Franklin, you need to have Smoke on the water and sometimes, the guys from the shop tells you that the delivery man has problems with the justice and that you need to do the delivery instead of him and sometimes the police is just waiting for you at the van location, and they often have an unmarked crown vic (I got the silver one yesterday using this technique)
That's all i have on this subject, hope i helped you guys and sorry for english mistakes i'm French