Car Break Ins

Leave absolutely nothing of value in your car. Thieves target rental cars because they know they likely have lots of goodies (and they know tourists flock to the Loire Valley). The trunk won't deter them. Leave your treasure in your room or stash them in a locker in the town's train station if in between hotels.

Personally I think that is over kill and the sky is falling -- But who knows what the truth is. We spent over a week in Loire Valley with luggage in the car but out of sight with no problems. Which proves only that we didn't have any problems. But the parking lots for the chateaus are pretty wide open and exposed generally with some type of light security so I don't think the risk is that high. Personally I would not worry about it to much as long as you take some reasonable precautions.

In the years we lived in Germany we had car trips all over western Europe, including the areas mentioned by the OP. Never had a problem with car break ins. Does this mean he'll be as lucky? Dunno.

At least don't ever leave anything it would be tragic to lose -- ID, money, cards, your computer, your prescription meds -- carry those with you for lunch or a tourist stop. Then if your car is stripped, it is just 'stuff'. I know people who lost everything at a beach stop including meds.

The problem with parking and visiting a tourist spot is you have no idea if it is notorious for rings of thieves who target parked tourist cars. It doesn't happen often, but be aware of what you can live with if it happens to you.

It's kinda like lightning striking. You don't know when it will happen and who it will hit, but it does occur. On our drives in France the parking lots in many places we visited had warning signs in several languages about break-ins so I figure they wouldn't put them up if it wasn't something real. I agree with the other posts that say don't leave anything you absolutely don't want to lose (passports, credit cards, cameras, etc) in the car, even out of sight. Also we made it a practice never to open the trunk in the parking lot. Why show any potential thieves which car actually has something in the trunk? If we needed something we'd stop a mile or two before our destination and get it out then.

If you're still planning the trip then another thing you can do is minimize the number of one-night stays. We like to set up a base for 2-3 days and do daytrips from there. Luggage is back in the room, not in the car.

You are a clear target. Thieves note everything about rentals cars, from stickers to department of registration to the fact that rentals have no floor mats. Don't leave anything in your car you do not want to lose.

I agree with the cautions about not leaving "can't lose" items in your car and not displaying your wares to possible thieves in the middle of town. I also like the idea of staying 2-3 nights in one place, just so your belongings aren't in the car every single day.
We've driven a lot on each of the 5 times we've visited France and stopped at numerous tourist sites. We follow the advice just mentioned and have never had anything stolen from our car. As others have noted, that just means it hasn't happened to us, not that it can't happen.
Of course it would be awful if it happened, but lots of bad things "could" happen while you're traveling. I think you should take reasonable precautions, then go and enjoy France.

I buy local newspapers and throw them in the car. Also, turn off, or disable the bluetooth, in all bluetooth enabled devices that you may leave in the car. Thieves will use blue tooth to find valuables. Never open the trunk at your destination if you have stuff in it. You don’t know who is watching.

Mo makes a good point about not opening the trunk. American city dwellers know this, but suburbanites may not. You should get out of your car as if you are arriving at work-no dawdling, strapping on armored shoulder bags, and pushing stuff under the seats.

I have rented hatchback cars that were missing the luggage bay cover. Pre-printed on the contract. We were lucky the day we were in the Loire with luggage. Other days the car was empty, of course.

Rick has mentioned tipping Bogus Traffic Wardens, but we didn't see them in the Loire.

Chiming in on not opening your trunk. Make sure everything you will stash, and everything you will carry are well sorted out many miles from your destination. Park and walk out of your car ready to go. The biggest advertisement you can make that you have stuff worth stealing is to be moving that stuff around in a parking lot.