Attach the driver to where he is now. Make him play the sitting animation and perfectly synced with the direction of the vehicle. Add all the regular actions, "Engine off", "get out" when still, "eject" when moving. Lock the car.
Add on key press eventhandlers for W,S,A,D and while holding shift etc for velocity additions.
OH ITS ALMOST PERFECT... but then the sound won't work properly... damn so close
</sarcasm>
There is nothing you should do, its a OFP/ArmA/ArmA2 bug that always occurs, as mentioned above.