I rarely use the rear seat in my Rav4 for passengers. It's folded flat most of the time between carrying the bike, a dog crate...or cargo from a trip to the grain store. I have a large, heavy-duty tarp in the back that protects the car material and easily shakes off any debris that may accumulate. It's worked out well for me.
1990 Univega Alpina/(stock) gel saddle
2009 Specialized Dolce Elite/BG Lithia saddle
2009 Jamis Coda Sport/Selle Royale Respiro saddle
2010 Jamis Aurora/Jamis Touring Sport saddle