It is a permanent extension the vehicle bay can craft on any vehicle's tray if 4 adjacent slots arranged in a square are free.
Once crafted, the vehicle will actually possess 8 standard slots where it possessed only 4 before installing the storage. Vehicle storage cannot be installed on other vehicle storage.
Do not mistake this Vehicle Storage with the Storage. Both have very different purpose, they are not the variant of the same item (as with battery for instance).
If you put this on a rover, you will no longer be able to drive it. This is meant for trucks primarily, however, if you have a second vehicle, you can use the rover as a trailer for it by attaching with a cable.