I'm pretty sure Car Parts can be recovered from various ground UFOs like couriers, humvees and armored car patrols.
adding them to the Stranded Govt. Agent missions (ie Car Wreck + 2x Govt Agent) would make sense, as would giving Car Parts alongside the Male Path -> Trucks research like the Wrench is currently. adding Car Parts to other terrain spawns such as tractors and garages would make them too common imo.