As an electrical engineer with a lot of experience in similar issues, I think you are mis-intpreting what Bruce said. Capacitors will not increase the voltage, however they can supply a large slug of current for a VERY short period of time. This will often enable a system to "ride-thru" a very short drop-out of the battery volatge due to either a flaky connection vibrating loose, or similar. However, my bet is that this capacitor is an attempt to deal with line noise. Whether it solves either issue is dependent on the value and type of capacitor. Larger electrolytic caps will help with the carry-thru behavior. Smaller non-electrolytic caps do a better job of fixing higher frequency line noise. In the automotive elcectronics world we often use bothe types of caps for wide range coverage. Frankly, in most cases, these sorts of caps are band-aids to cover issues rather than good design practice. That said, they usually dont hurt anything. However, electrolytic caps do tend to dry out and fail, especially at the high temperatures that could exist under the deck of a boat sitting on a trailer in the sun.
That's not what I meant. I am a EE so I know what a capacitor does in a circuit. I was talking about an instantaneous voltage drop where the inductance and resistance of the power feed was causing instantaneous drop due to a high frequency demand. Like I also said, the size of the cap would also tell what they were trying to fix here.
