Classic car alarm systems?

If you are going to install an alarm, look at these first. You can add options like glass breakage and jacking/towing and use your smart phone etc. Not as cheap, but I think you might need to get more serious as they will be back. You don't want a cheap system.

Viper - Car Alarms | Remote Starters | SmartStart | Window Film and Tint | Vehicle Security | Keyless Entry

That said, an alarm system probably won't stop them once they figure out when you aren't around. Backing into a driveway with a trailer is very ballsy... These guys are pretty serious and want your car. This wasn't a crime of opportunity. An alarm will make some kid trying to steal your radio run away, but not these guys.

You might want to look at other alternatives to make it more difficult for these guys to steal.