Interest can be quite enough; I'd also say that GM's is the exception instead.
Generally speaking, the majority of online advertising is geared toward interest in the first place because interest leads to intent. Facebook is earlier in the customer acquisition lifecycle as you can grab their interest before the intent, and then they'll go directly to you instead of searching.
This is why Google in general has been so scared of Facebook (not that I think they should be). What is happening is that people are figuring out ways to cut search out of the loop.
Take a look at all those online games you see on FB ads now. I can tell you that most of the MMO ad budget now goes to Facebook as you can easily get people to download the game and start playing, whereas trying to pick people off search for MMO's is obscenely expensive. As search saturates, marketers tend to move towards the point of greatest ROI, which happens to be in the interest phase.
Cars are a tough sell, no matter where you try. I'm not saying that every single type of product works on FB, just that we really should not compare FB to AdWords in the first place.
"Prove the rule" is a colloquialism that means "test the rule". The exception ends up disproving or clarifying the rule, not verifying it. It doesn't mean prove like passing that test.
Actually, "the exception that proves the rule" means that a recorded exception to a rule can be used as proof of the existence of an otherwise undocumented rule. It dates back to medieval legal terminology.
Go watching a movie are such an ingrained part of life for many people and thus we are always "on the look" for a good movie to go watch.
But try and do that with a car brand or any other non-commodity and intent suddently becomes the most important factor.