The amount of learning we do each and everyday is pretty astounding.   We learn through successes and through failures–you try a “shortcut” only to find it takes twice as long, you try a new recipe and it’s a huge hit, you try to break up with someone via text message and learn how much of a BAD idea that is, you get stung by a bee only to find out you are highly allergic, you wake up to find a dead fish in your fish tank and you learn why it’s important to check the temperature.  The lessons we learn through life successes and life rewards, I think, are some of the most ingrained.
I get quite a few comments by people various places when I’m unloading my dogs from the car.  I can swing open the doors as wide as possible and neither dog will get out of the car.  I’ve watched people struggle to get the leashes in their hands before the dogs fly out from the car and take off in some direction… these same people frequently  look at me with amazement as my dogs stay in the car with doors open.  I travel a lot with the dogs and one of my concerns is having one of the dogs get loose during a potty-stop.  So this behavior, of respecting the boundary, is very important for me … and is a behavior I think ALL dogs should learn.  Having dogs who will not push their way out of a car is really about safety.  It provides you the time and ability to put a leash on the pup or grab the leash before they leap out, you can open a door without the dog squeezing out and perhaps getting hit by a car or lost in the woods, and it gives you the chance to start off with a level of control and focus at exciting places.
When people ask about it (and once they get their dog back, they inevitably do), I tell them it’s a very easy behavior to train!  It’s about knowing what the dog wants and using that to your advantage.
When the dogs are in the back of the car and we are at the park, I know what they want is to get out of the car and go play.  I will use getting out of the car as a reward for staying in the car.
I tend to start by working in lower distraction and safe areas (a garage is preferable because it’s very low value… they will probably not be itching to get out in the garage)–this is the only time I will use food rewards.  I cue the dogs to hop in, I shut the door, wait a few seconds, and then reach for the door handle–if the dogs get smooshed near the door, I turn around.  If they are just casually waiting, I unlatch the door–if the dogs try to push the door, it gets latched and I move away (15 seconds).  If they aren’t too pushy, then I open the door just and inch or two, if the dogs rush the door, it’s carefully shut and I walk away (for about 15 seconds).   If they stay put, I continue to open the door wider and wider…. if they make a move to go through the door, I shut the it carefully. If they do get out, I put them back in and shut the door (no yelling, just back inside).  When you are able to open the door all the way (even if just for a moment or too) give your release word (okay, free, break) and toss a treat on the ground if they need motivation to get out.   Don’t use any cues  to make them stay in the car, don’t say “stay, stay, stay” or “wait, wait, wait”… we want the behavior to be a default or expected behavior, not based on our verbal cues.

Once you are able to open the door normally with the pup staying inside, I like to take it a little further while still in the low-interest area.  With the door open, I move to grab the leash or move to hook a leash to the collar.  If at anytime the pup moves to exit the car, the door shuts and i walk away for 15 seconds.  I work up to being able to gently tug on the leash with out them exiting the door until I give the release word.  By this time, I should be able to give the release cue without tossing the treat but with rewarding with one afterward.
This is pretty much the same process used repeatedly starting in new places.  After the garage, I would go to the driveway or street in front of the house–some place medium to low interest.  I would put the pup in the car while in the garage and actually drive it into the driveway or street then stop and work.   You can even drive around the block and park back in the driveway to work.  I don’t like to take this training to the next level until they have been successful for a while (setting the pup up for safety and success).  When they are good in the garage, driveway/street, then I would take it to the next level.
Take a short drive to a place they go somewhat regularly (so it’s not an added level of excitement of a NEW place).  Be safe about things… if it’s a high traffic area work on the side of the car that is less busy.  Repeat the process from the garage… if at anytime the pup tries to push its way out or gets too close to exiting, shut the door and walk away for 15 or so seconds.  Work to the door being open and you grabbing the leash (for safety you can get the leash with the door not completely open, but open the door fully before releasing the pup)… if you can get the leash and open the door, release the dog, no food and go for a nice walk or do that activity.  If the pup jumps out before released, calmly put it back in the car shut the door and turn away for 15 or so seconds.
If you want, you can up the criteria to requiring eye contact before the pup gets out of the car.  You can add criteria by requiring eye contact AFTER the dog leaves the car.  Again this is all about having some focus/control before starting an activity.
This is one of the first things I worked on with Rio because I think it’s such an important safety default behavior.  I don’t worry about him leaping out of the car to chase animals, or to run to a new place, or to just get out and wander into traffic.  What I’ve done is up the criteria for eye contact… they need to give me eye contact before they can exit and they have to give me eye contact within 30 seconds of being out of the car.  When they offer eye contact it initiates the fun.  I’ve used this method to build door manners and crate manners as well… no more rushing out the door, no more rushing out the crate… teaching the pups to show a little impulse control.
The training isn’t hard… it’s all about real life rewards and life consequences and in my experiences, dogs tend to pick it up quickly.