Complex sentence: The body had a complete 12-volt, negative ground electrical system with a concealable 4-beam headlight system, including brake, back-up, marker, instrument, and interior lights.
Split into simpler sentences: The body had a complete 12-volt, negative ground electrical system. A full set of lights were installed, including a concealed 4-beam headlight system, brake, back-up, marker, instrument, and interior lights.