My first thought last night, was that perhaps the car had an inside switch for openning the trunk (like, located in the glove box). But they are saying the car had been sitting for 3 months, I have to wonder, if there where such a switch, would this model require a working battery for the juice to operate it. Maybe someone knows a bit about this make and model of car?
I saw one of these latches in a late model car that I rented recently. It's mechanical; you pull and the trunk lid pops. But when they began to be used, is a different question.
I'm almost certain there is a lever on the left side of the drivers seat which opens the trunk. It is mechanical, not electric. The boys would know how to use it. The bigger question is, if they got into the trunk without a fourth person, how did they close the trunk on themselves? I'm sure it can be done, but it wouldn't be easy.