In Chapter 15 Part 2, when I go to the military truck near the radio mast, the message is "I don't..

Have you already seen and tapped on the open window in the hangar building? You may need to do this first. This lets you know what the truck is needed for and should activate the use of it.

For more questions for Tiny Room Stories: Town Mystery check out the answers page where you can search or ask your own question.

You need to find the ladder hanging from the roof with the solar panels first, then the truck message will change