A Mobile Supply Unit is a vehicle upgrade which allows a vehicle's trunk to serve as a resupply point. To use it, you walk up to the rear of the vehicle and hit T, which will begin the resupply process, replenishing your ammunition and grenades.
As you begin to level your character, you will be presented with an email that will allow you to choose this item, or the Field Supplier Kit.
Unlocked for purchase at Arms Dealer - Grade 10.