You can use addVehicleSirens and setVehicleSirens.
bool addVehicleSirens ( vehicle theVehicle, int sirenCount, int sirenType, [bool 360flag = false, bool checkLosFlag = true, bool useRandomiser flag = true, bool silentFlag = false )
bool setVehicleSirens ( vehicle theVehicle, int sirenPoint, float posX, float posY, float posZ, float red, float green, float blue, [float alpha = 255, float minAlpha = 0.0] )