You can access a vehicle's trunk by standing at the back of a vehicle and pressing G. For some of the oversized or oddly shaped vehicles it may take a couple tries to find the sweet spot where G will open the trunk. The number of inventory slots in a vehicle's trunk depends on the type of vehicle. Smaller vehicles will have less slots.
You can access a vehicle's glove box by pressing G while sitting in a vehicle.
You cannot store cash or inked/marked bills in vehicle trunks or glove boxes at this time.