Vehicle GPS Tracking

Does anyone have any experience, which are the latest after-market vehicle theft recovery (tracking) systems that also get good reception and work on a convertible? The mounting of the antenna is particularly problematic on older (classic Mercedes) convertibles because the steel dash shields the antenna and other locations, either don’t provide enough space or create other problems.

I would like to be able to not only track the car on-line, but also by calling in to check for its current location (for instance, when I am parked in a parking garage in the mall, etc.

I am not interested in LoJack or Teletrac as they do not work well. Also, I would like to be able to track the car myself via internet.

My main problems with the past systems that I have used is that they often did not work, especially when the car is near an airport, inside a garage or in other "dead zones".

THANKS!
Tom