Like in the wiki, to be exact.

I think this is what happens:
If you have the stock player there (/usr/bin/mediaplayer) and none of the players is running, making a D-Bus call will open the stock player. However, it has no idea what to do with "open_mp_car_view" and does nothing. Neither exits nor shows the main window, but it's still sitting in the background. Because of that, all consecutive calls will reach the stock player and there will be no visible effect, even with OMP running.

Try this way:
1. Make sure that the stock player is not hiding somewhere (execute "killall mediaplayer", for example).
2. Open OMP.
3. Call open_mp_car_view.
4. If it works, the above paragraph seems to be correct.

Conclusion:
You need to either launch OMP prior to opening the car view or make OMP (/usr/bin/openmediaplayer) your default player.