On the 0.56 version we are unable to enter trucks. You CAN access the front hood but not by opening from the regular place. This has happened on two different occasions with 2 different trucks. I will provide screenshots with location/type and I also uploaded a video with the issue you can see on the following link:

Screenshot1: http://i.imgur.com/cNS0p9m.jpg - Truck not accessible
Screenshot2: http://i.imgur.com/0gNsvnh.jpg - Truck not accessible
Youtube link of attempting to enter newspawned truck:
https://www.youtube.com/watch?v=-AebFiTCEOs&feature=youtu.be

PS: On another side note, in the video you can see the engine spinning despite the truck being completely shut down :).

*OBSERVE!* While uploading the video I found a workaround in how to find the doors. You can look at the back of the truck and scroll with your mouse, that way the options for car doors will be available. It MIGHT require a relog but I am not sure. Here is a screenshot of how I got it open after scrolling with mouse: http://i.imgur.com/V8RmF9N.jpg