Everything was installed, doors, hatch, wheels, radiator, battery except sparkplug. I had sparkplug in player inventory. The car had lots of gear stored, took up most of the 300 storage space. Had a full gas container of gas put in. Radiator was full of water... as soon as I entered the drivers seat the car went under the map moving up and down like on a trampoline. Eventually it stopped and was able to drive it a short distance. Ended up stuck on a small Bush, tried emptying all stored contents from the gunter apart from 5 items that refused to leave the car. They wouldn't go into players hand or inventory or be dropped to ground. Maybe they weren't even there, maybe someone had taken those items I'm not sure. I know someone had been there though and opened the doors that I left closed... really annoying bugs.

Seems the weight and inventory capacity have something to do with it. Spent three days collecting all required parts only to leave it, given up on cars now... I'll wait till the next update.