Complex sentence: He worked on the Guard unit's staff car, which was a hand-me-down Cadillac left over from World War I. Darnell also participated in weekly drills and summer camp with the Guard.
Split into simpler sentences: He worked on the Guard units Staff car which was a "hand-me-down" Cadillac left over from WW1. Darnell would also participate in the weekly drills and summer camp with the Guard.