They are an important asset of any vehicle, and the vehicle will not run without them.
They can also be used in Generators, such as the one on the Airfield and the one in the Hotel in Hayward Valley.
Use
Open the inventory by standing next to the vehicle, and slide the sparkplugs in their respective slot.
Loot spawns
Spark plugs can be found in hardware stores, warehouses, garages, and most technical buildings. They are quite small to the eye.