We bought a Nissan at the dealer because we wanted a vehicle with an official service presence on the island and VW's didn't meet our needs or budget as well.
Ours is registered with a CURP because we're permanent residents. Gisela Rodriguez is possibly the only lawyer who could handle registering a car without a CURP. I have heard no reports of anyone else's managing this feat. There may be other good reasons to seek permanent residency if you own property on the island besides registering a vehicle.
Our property manager drives the car on his errands about once a week to scare the snakes out of the fuel injectors or whatever, keep the battery going, etc. He delivers it to the airport before we arrive and picks it up after we leave. If you go with cvchief's suggestion, you will be much happier in the long run.
We rent a parking space downtown. Our house has iron gates, but they wouldn't prevent someone from climbing over and messing with the car. Another reason is that we rent the house out and we don't want the car there when renters are there. Finally, the parking space rent is low and it gives us "free" parking downtown. This is particularly nice when we use the ferry and return after the public lots are closed since our is accessible 24/7.
Irma Portilla de Cantarell is our insurance person as well and we've been highly satisfied with our insurance.
Honestly, unless you get a great bargain on a car, rental fees are still likely to be less than the cost of ownership for 5-6 visits per year. It is nice, though, to be able to leave a "beach bag", tools, sunglasses, and other personal items in a car and to have it magically appear at the airport for your arrival.