Well in the script you would first need to loop through the map file with all the vehicles being spawned,
while looping through it, you could get the positions of them, store them in to a table and destroy the vehicle.
After that's done, when ever a player joins the server and they're the police then use this:
local x,y,z = unpack(tableName[math.random(1,#tableName)] --The table would basically look like this: local tableName = {{x,y,z}}
to get the random position of a police car, after just send them to that position and warp them into the car.