What if the thing that the thieves want is the vehicle itself?
I have VW Vanagons.
The value of these things is pretty absurd and generally they are always full of stuff.
Vanagons are pretty easy to break into and pretty easy to get running without keys.
Many thieves will target these vehicles specifically for those reasons.
Pop a door or bash a window, jump in, drive off and take your time sorting through it all at a leisurely pace.
Then with the right connections sell they van to someone else.
I want protection against people simply gaining access to the inside to take the whole thing.
Pretty tough to to keep the whole van out of sight when you're parked in town.
Read the thread I posted on the first post.
Sarcasm really won't help as much as common sense, so don't park your vehicle in what you consider is a shady spot.
The principles still apply, thieves won't take what's hard to get. If a thief considers your vehicle to be harder to take than the next, he will move on to the next. All you have to do is make it LOOK like it will be harder.
As for effective mechanisms for protecting your whole vehicle, we usually have combinations of the following physical and tech measures:
1) Steering wheel locks. You know, grandma's red bar on the steering wheel. Ancient, but very visual and effective. Thieves will see that it will take more time and effort to steal your car, so they'll move on.
2) Pedal or shifter locks. Some pedal locks hide all three pedals in a folding vault style lock, other simply use a padlock to join brake and clutch pedals together, making it impossible to shift the vehicle or drive it correctly. These aren't as visual, but they are effective once they try to drive off.
3) Kill Switches, have to be wired in differently depending on your vehicle and hidden appropriately. You can wire a killswitch in any sort of engine.
4) GPS Trackers. Hardwired to the vehicle, will help you recover it once its taken. Some of the good ones, like the ones we use, have built in killswitches. You can shut the car off from your phone. Very, very effective. It has saved us a few of the company's trucks working in some of the more dangerous places. We usually follow it real-time and shut the vehicle off in front of police stations, checkpoints or something similar.
Some of the simpler features, like a cheap alarm, will also work. Specially ones with the little indicator LED flashing in the dashboard or windshield, indicating the alarm's armed. Stickers stating that the vehicle is protected by alarms, GPS Tracking, or similar, also make thieves think twice. This can add to a long line of etceteras but you get the idea.
I can't say this enough: The thief won't take what's hard to get. It will move on to the next just by making it look like it will be harder than the next. Given enough time, none of the security measures mentioned will last. If he wants to take your vehicle and has enough time, he will regardless of how much you can throw at it.