The capacitor will power electrical components such as the air conditioning and stereo, which are normally powered by an alternator connected to the engine.
As eardrum vibrations naturally create a usable voltage from brain signals, the prototype can build enough charge in a capacitor to drive a very low-power wireless transmitter that relays the electrochemical properties of the ear and potentially diagnoses balance or hearing problems.
But capacitor banks tend to be very large and hard to site near customers (who love to guzzle power but complain about new power lines or hardware in their neighbourhood).