Our fellow posters from Australia bring up several excellent points about car theft which may be a bigger problem in France than is generally known among many tourists (my French friends caution me about this all of the time).
It is important to be careful, keep in mind that thieves know which cars are likely being driven by tourists. Those with:
1. Red license plates - indicating a non taxed or foreign leased auto.
2. Distant department numbers on the license plate. The last two digits on a French plate indicate the department of registration. Europcar for instance registers many of their automobiles in Haute Normandie. When driving around Provence with a plate ending in 76, the fact that you are either from a distant department or probably a renter is not missed by those looking for tourist prey.
3. Little white square stickers still stuck near the refuel port which say something like "gasoil only". These stickers are placed on all new cars with diesel engines to remind owners of the correct fuel requirements. Real owners remove them, renters don't bother.
It is difficult to avoid leaving items in your car when on vacation. Some suggestions:
1. Lock valuables in the glove box.
2. Leave nothing in plain view, particularly maps and English language magazines or newspapers.
3. Place a copy of a local French newspaper (date not important) clearly visible in the windshield.
There are no guarantees but tourists can be easy prey for the unscrupulous. Common sense can help avoid tragedies.