How to unlock
Drive 200 meters with the Type A-469 on the back of your truck.
An easy way to do this is to park a truck with a Carriage attachment just below the dip in front of the garage on the Coast Map, then drive the A-469 up to the back and winch it onto the truck bed (while controlling the A-469, so you can correct any steering problems). Then just switch to the truck, attach a front winch point to one of the front winch points on the A-469 to keep it in place, and drive off. You'll take damage, but you'll most likely make it 200m without any problems.