jcolman: Have you tried mounting the rig towards the rear of the car for longer exposures? Cutting down on the engine vibration may help. Finding a smooth road couldn't hurt either.
I haven't had time to do much of anything but the shots you see. I just finished the rig yesterday. You're right in that I need to find a smooth section of road and turn the engine off. These were simply test shots in front of my house to see how much vibration I was getting.