Leaving luggage in the car

I would still keep my valuables on me - passport, money, credit cards. My nephew had his car broken into in Israel - lost his entire wallet and passport :(

Most cars have a button that you push to open the trunk next to the driver's-side seat. It takes 2 seconds to break a window to get in.

I would leave my luggage at the hotel if the option is available.

Swap roles with the criminal for a moment.

Let's say you are hungry/broke/strung out/opportunist (pick one or more, or add your own) and wandering along you see an unattended car (probably with rental stickers) (even the little sticker near the fuel filler saying what type of fuel or a tiny bar code in the window) that has a blanket in the back which is kind of lumpy.

Might you wonder what is under that blanket? What would you do then?

I always liked Rick's comment that he doesn't park in places where he sees broken glass on the ground. Once you've taken that precaution, Nigel's point is correct. So I always try to get a trunk or at least a hatchback that has an integral cover. My next step, if concerned, is to use a cable lock to secure the luggage to the car. I often carry one. They are cheap, light and useful in a number of situations. It won't stop a determined thief, but it will force them to spend some time and effort which may be enough to discourage them. Lastly, we don't keep anything more valuable than our clothing etc. in the car when we're away. We use money belts and day bags. All that said, we toured the Irish countryside for several weeks in a rental car with no problems even though we parked for hours at obvious tourist spots like Powerscourt and Kylemore Abbey, and in some very lonesome places like out in the middle of the Burren. Take whatever level of precautions you deem sufficient and then relax. Have a great trip.

We toured around for 2 weeks in July and August without any issues. We never leave anything of value in the car.

you could always dump a garbage full of rubbish on the dash board eg: empty chip packets, local newspapers etc and make it look like a local 'builders/tradies' car. seriously sometimes you don't have much choice, when travelling from place to place, waiting to book in to accommodation.
we carry a 'mobile' safe. pacsafe mesh bag that we can if needed lock valuable things out of sight and to something strong, inside the car and/or hotel room etc. it gives us peace of mind, if nothing else.

Thanks folks. I should have added that we always carry our valuables with us in money belts, etc., or lock them in a hotel safe.
There are times, though, when going from town to town, where we might stop and have to leave our bags in the car (without the valuables). All anyone would get is a mixture of clean and dirty clothes, but we would prefer to keep even those. Sometimes it's difficult to find a car with an actual trunk at a rental agency. I agree with Nigel that the blanket idea is poor at best.

Most of our rentals have been hatchbacks and there are times when luggage stays in the trunk. We bought cable locks at Home Depot & cable luggage handles together & try to attach to a handle or the like to part of the car. We have been victims of theft of clothes from B&Bs--nice ones- cleaning people probably---so are very security conscious.