A guy I know squeezes his homemade mando-bass (still a large instrument) into a Karman Ghia. Only he knows the magic twists and turns to get it in and out.
I had an old 86 Honda CRX (two seater), and figured out how to fit my bass, amp, and two cellos. I'm pretty sure I couldn't do it again. The main trick, though, is to recline the passengers seat and sit the bass in it.