A couple of suggestions that may or may not work for you in a given state, but worked for me.
1) Microfiche lives forever. Computer records usually have a limited life, and DMVs will *always* say that they only keep their records for X years. And I'm not saying that every state has such an office, but every time I hit a dead-end, but was politely persistent and pushed up the chain (you need to be on the phone for this), I was eventually directed to a microfiche storage office that had all back records. It helps if you specifically refer to microfiche (pronounced micro-fish) records, until you find someone that knows that they are. In some states this office seemed to consist of only one person n the state, and most of the people you interact with at a given DMV will not even know they exist, so you need to push up the chain of command a couple levels until you find someone that has been around long enough to know where to send you.
2) If you hit a privacy law, you might try to argue your way past a complete dead-end by saying that you really are looking for the earliest reference to a dealer, and don't need to see names or addresses of individuals, and that it would be okay for them to mark through that info with a magic marker (which you can usually see through anyway if you hold it up to the light just right).