This could be a rusty armature or worn brushes problem in the fan motor..
What happens is the armature gets rusty and the brushes can't make good contact with the armature.
With the truck moving, there is enough vibration for the brushes to make good contact with the armature.

To test this theory, turn on the fan and tap the fan motor with a hammer, see if fan motor starts to spin.
If it spins there could be two problems, the brushes are worn or the armature is rusty.

The fix is to ether replace the fan motor or sand down the armature with emery cloth to de-rust.
If brushes are worn you could replace brushes.

Note: Some fan motors are hard to take apart.