theft from auto in southern france

My husband & I learned a hard lesson. We landed in Dublin, Ireland and was heading across country to Galway. We stopped on the way at a sight about an hour north of Dublin, parked our car in the lot, and took a tour. When we got back, we realized someone had broke into our trunk and stole all our luggage. I was devastated. I felt there went our trip. That first day was totally ruined as we waited for police, made a report, and then drove to our intended first overnight stay. Stores were already closed when we got there so had to sleep in our clothes. Next day, spent the morning shopping for everything...suitcases, clothes, toiletries, etc. But from then on, everything went very well. The people of Ireland were sooo gracious and helpful. Despite our terrible start, we still fell in love with Ireland. Also, the police kept in touch with us each day to let us know the status of our luggage. They met us back in Dublin before we flew home to give us back my luggage that they found (everything still in it). But they never found my husbands. It appears they had problems with the young kids stealing from cars in hopes of finding cash or electronics. If they didn't find anything, they would toss it to the side of the road where mine was found.
Suggestion, do NO sightseeing with anything in the car...not even in the trunk.

Southern France has been mentioned in some travel literature as a place where car break-ins may be more likely.
I have no personal experience being the victim of theft from a car in Southern France. However, I've always used secure overnight parking, provided by the lodging, when there and other locations.
Putting things in your trunk, and leaving a local newspaper showing in your car, apparently doesn't work--they're onto that one.
Trunks are easy to break into.
Secure overnight parking is the key. There will be times during the day when visiting a tourist site, that you'll just have to go for it and leave things in the car--in that case, leave nothing visible (if possible), carry anything with you that you can carry (especially vital stuff); and if there is anything you can't carry with you, put it in the trunk and try to find a parking place where you can back the car (the trunk) against a high wall.
And don't park where you see broken glass on the ground (as someone else already mentioned).
But you can only do so much, during the day.
But at night, you can obtain (and pay for) overnight secure parking.

When we stayed six months in Provence the rule of thumb from the locals was to never leave anything visible inside your car. So we did as they do and put everything in the trunk whether at a shopping mall or a remote location. Therefore, we had no personal experience of theft but do know someone who left his camera visible as he briefly stepped away from his car for a panoramic view--door unlocked, I believe--and swoosh, the camera was gone.

I do personally know a traveler who had a car break in within the past couple of years in southern France (and who really knew better than to have left things in the car). Note that car rental CDW only covers damage to the car, not your belongings.