[Jewell]

thats true, vehicle Full health is 1000. but even we can set any vehicle health value (like 5000 or more) .

pawn Код:

public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
   if(GetVehicleModel(vehicleid) == 596/*|| GetVehicleModel(vehicleid) == other police car ids */) 
    {
        SetVehicleHealth(vehicleid,5000);
    }
    return 1;
}

improve above code yourself for your script. (you should block vehicle health refill when player re-enter the car)