Luggage in trunk

I'm interested in this subject because of travel in France this summer. RSE France book says to toss a French newspaper into back window of car, and have nothing visible. You could apply that to Italy, I would think. People are breaking into cars all over the world. (Beware of Electric Beach, Oahu, Hawai'i). My plan is to leave luggage in trunk while touring France, nothing visible in car. I'm also thinking of carrying Rx meds, headphones and cell in small backpack that I will keep with me at all times; I wear a backpack backwards in congested areas no matter where I travel. Easy to buy clothes, are hard to replace electronics and medications.

I traveled a lot in my life. Over my lifetime I had my bag stolen from the trunk only twice. Once at the Cinque Terre over 35 years ago when I lived in Italy, and once in California, about 5 years ago. I lived half of my life in Italy and the second half in California. So the lesson that you can learn from my experience is that it is not a common event, but it can happen anywhere.
There are some strategies that you can employ. One is not to have luggage visible inside. So make sure they give you a car with cargo cover (most compact vehicles in Europe are hatchback). Secondly if there is one, park in a guarded pay lot. Street parking and secluded lots are less safe. If you can't find a proper lot, park in a very busy street with lots of pedestrian foot traffic or in front of a sidewalk cafe, or busy shop. People are less likely to smash windows if there are lots of people walking around near your car. Finally, if you can, back your car against something, a wall, a tree, a bush. Make it as awkward as possible to open the trunk.
Then there is another strategy an Italian friend of mine used. Instead of packing the suitcases we would put them in open. Then he would throw all the clothes loose on top. His trunk would be a cluttered mess, but he thought that it is much easier to grab a full bag and go, than to gather a bunch of messy loose clothes. Thieves have no time to pack your clothes in the bag and take it away.

I have done this in most countries where I have driven in Europe, probably eight or nine countries. (England, France, Ireland, Germany, Portugal, etc.), but I don't know if I would do it (leave luggage in the trunk en route) in Italy, much less Sicily. (But I would not drive in Italy at all, either). It a difficult dilemma because not having to backtrack saves a lot of time. I would probably do it selectively weighing the costs and benefits of each situation, and making sure to keep anything remotely of value out of sight. Pack a small bag to make sure it fits in the trunk, trunks on rental cars are getting smaller and we had to switch to a 22" bag. Some countries like England have a high prevalence of security cameras everywhere, which tends to discourage thieves. Remote areas unlikely to be monitored where there is no one around and you walk down to the beach, those places would be a higher risk.

If I count the times my cars were burglarized in California, vs the times they were burglarized in Italy, California would come out as the more dangerous place. Although they took my bag only once, however my windows were broken 4 or 5 times, often just for the change in the ashtrays. In Italy my car (or my parents' cars) were open only once in Riomaggiore, and they didn't even break the window or the doors, proving that Italian thieves are more skilled too.

Perhaps your car might be less of a target by making it look unattractive to thieves. One example, is to put what appears to be a soiled diaper in clear view.

I agree with Roberto's suggestions ....ok throw a fake dirty diaper on top too

You guys are evil. Also clever.

It just so happens i have NOT had rental cars broken into in England , Ireland or Italy . Kansas city, YES, behind an ice cream parlor at 3pm. Lost prescription sunglasses & a 30 dollar camera from the console , my suitcase & my briefcase (no electronics, but my calendar for work which made doing my taxes that yr a nightmare). So yes, i should have had stuff locked in trunk....street value of what dissapeared was maybe 200$ plus the sunglasses, but broke my driver window out.....driving home a couple of hours minus a window was a real treat (happened on a saturday evening , i HAD to be back to work at 8am sunday)

You probably already know this, but be sure that you have what you need from the trunk long before you get to your parking space. Don't open the trunk for any reason!
And, yes, leave an Italian language paper or magazine visible but not a map!

The archeological site at Selinunte has a carpark. There is a public carpark about 200 meters away, with an excellent restaurant in the middle of the carpark.

Thank you everyone for your helpful responses. And good to get the information about parking lot and restaurant at Selinunte.