I think its working.. This is what I did.

Right click on mini map icon select bar edit.
Right click on bar and checked possess.
Left click the bar to change to to blank bar titled possession.
With my mouse over the bar I typed
/mac add for 6 buttons.
I right clicked on the icon again to select bind edit.
Moused over each button and set the bind to 1, 2 , 3, 4, 5, 6

Now when I am in a vehicle it shows the buttons for the vehicle on this bar.