What do you use the truck for on shrink ray island?

The truck, other than lending its battery to the TV remote, has only one purpose: to get you out of the apartment when CJ is in danger. You can make a ramp for it by shoving the book Tess's Tree off the top of the bookcase. But you cannot use the truck until the proper time, after you have completed the other steps in the apartment and after you receive the second of 2 Morse Code messages from CJ.