As a foreword, let me say that this is not an easy process. Your success depends on several factors. How much information did you keep about the vehicle? Do you have any photos of the vehicle? Do you have the contact information of the person you sold it to? Does that person have the contact information for who they sold it to, and so forth?
I recommend starting with posting your information here, and at my twitter First Car Reunion at Twitter I'm doing all I can to help others with their search. If posting here, be as specific as you can, and provide a link to pictures. There are dozens or free web hosting services. Post your photos there for easy linking.
If posting at My Twitter give the Year, make and model, and prove a link to a site where you have a full write-up on the vehicle. Again, there are hundreds of free website creation sites. It doesn't need to be fancy, just needs a description and pictures.
The single most valuable piece of information you can have is the VIN number. That's really the only way to be certain it is in fact the same car you parted with. Second is pictures, this might help someone identify it if the overall appearance hasn't changed too much. Then there's where it was was last seen and who you sold it to.
If you don't have the VIN number, there are a couple of ways you might try to find it. This mostly depends on how long ago you owned it. If it was 20 or more years ago, chances aren't good you'll be able to get it, but I truly believe it's worth trying.
-Contact the DMV of the state in which you owned it. Give them your information and see if they have any registration or title records for the car. If they do, they'll be able to give you the VIN number.
-Contact the Agency that insured your car. I've heard stories of agencies keeping records well over 10 years if they've been around that long. They may have the VIN number on file with your policy information. If you didn't have insurance for the vehicle, this won't help you.
Now, if you have the VIN number, these resources can help you.
Or a similar VIN searching service. This may not work for vehicles with the older style short VIN numbers. If it does work, it will show you where it has been registered and where it's currently registered. It won't tell you who owns it, but it will give you an idea of where to look.
-Check My Ride
This has the same Caveat that carfax does. Old style VIN numbers may not work. I had good results with this service. It's free and was able to give me a lead on which town my vehicle was last seen.
-The Lost Car Registry
This is by far the most comprehensive free service available. You don't necessarily need the VIN number, but it helps. You create an account, then you can search vehicles that are already listed and create a listing for your car if you don't happen to find it. This site already has several success stories, and as it becomes more popular I'm sure those numbers will grow.
- Your State DMV
Because of Privacy laws, this is a long shot. If you happen to know someone that works for them, your chances may be better. If this resource does work for you, do not divulge how you found the car. This will create a world of hell for not only you, but the individual who helped you.
Don't bother with this route unless you have a friend in the justice system. Law enforcement has access to a few databases which will allow them to search a VIN number. Again, if it does work for you, do NOT divulge how you got the information. This will create a lot of problems for you and the Law Enforcer who was kind enough to help you.
Personally, I don't have the money for this option. If you can afford, and have the VIN number for you car. You have an excellent chance of finding your car. PI's are able to bypass driver privacy laws and access the ownership records that you cannot. They should be able to provide you with the name and contact information of the current owner.
If you don't have the VIN number for the car, all is not lost. There are still a few things you can try. These resources are still helpful to you even if you do have the VIN number.
Look for clubs for your particular make or model of vehicle, or even the body style. I received several good leads in my search using The G-body forums as well as the National El Camino Owners Association (NECOA). There are almost certainly car clubs for enthusiasts of whatever you happen to be looking for.
Talk with the locals and hand out flyers about the car you're looking for. Ask owners of similar cars if they know of car clubs or resources you may not have found yet. If you know the general location of the car, go to shows there if it's nearby or within your resources. I've gotten a couple of leads handing out flyers and it only cost a few dollars to have them printed.
Look through their websites and magazines. If you have a particularly compelling story you can even write a letter to the editor and see if they will publish a story to help you with your search. If they have shows or vehicle tours, you can contact them and see if any cars like yours were registered. Hot Rod Magazine informed me that they could send a letter to the owner on my behalf if they did find my car.
Places like Craigslist and Backpage are great places to post an ad for your car in the wanted section. So far this has produced the greatest number of leads for me. There are also publications such as Classic Car trader where you can pay a minimal fee to post an ad in their magazines. If you have the money, it's well worth a try.
Keep in mind, there is every chance that you may come to find your vehicle was crushed or totaled at some point in its life. If this is the case, I'm sorry for your loss, but the silver lining is having closure as to what happened to your car.