I bought this restraint system for Trevor (size small):
http://www.champk-9.com/CCSS.htm
I purchased one harness and two restraint straps (one for each vehicle).
I looked at the various car seats but the one shortcoming they all seemed to have was that they all utilized your dog's own walking harness. Since most walking harnesses snap closed using only a plastic buckle, this would create a weak point, possibly coming undone, during a crash. This Champion system
seems to address that weakness. I just placed the order a few days ago, so it hasn't arrived yet, but I think it is going to work out well. Oh, and the restraint harness can be used as a walking harness, as well, once you reach your destination.
-Debi