my dog has grown up having a best mate down the road that he walks with regularly, she is a bearded collie, a lovely dog.
they love to meet up and run around playing chase etc. mine has stayed at hers and vice versa. today for the first time we thought we would put both dogs in my friends car, an estate, his dog got in but mine wasnt having it, even when i got in he wouldnt get in. i think he was frightened of the close proximity of the other dog, although she couldnt care less! or was it because it is her car, very odd. hes always trying to get in the postmans car and anyone else who comes to visit. They never fail to surprise you, in the end I had to walk!

I may not have been him at all.  She may have decided she didn't want him in such close proximity or just because it is her car and given him a signal too subtle for you to see but clear as day to him :)

You haven't said if your dog is used to a car? It could be that he knows it's not his territory and is wary of of a new experience. Must admit to having the opposite problem. Any car anywhere will do...

I would agree with Isabel on this one.  Bitches can be very funny about having other dogs up close and personal especially in their territory, one of the reasons when I would always take my bitch to the dog for mating.  She probably let him know that she didn't want him in her car and your boy was very wise to back off.

yes that makes sense, my boy adores my car and will stand by it waiting hopefully to go somewhere, she is more in control than I realised.

If going in someone elses car the owners dogs should be removed and yours put in first, otherwise your dog may feel too intimidated to get into their space, and the other dog is likely to just be ever so slightly guardy of it's owners car, but in my experience once the other dog is already in there before them then they are fine.

This is something my friends always did when taking someone elses dogs in with theirs, and I have found it works well.

I sometimes travel with a friend whose own dogs get quite guardy and won't let mine in, but after being removed and mine put in first all is well. 

They do know each other of course and are on good terms.