I'm trying to add a Beacon light onto the Barstow but it ends up like this, And I don't know how to fix it Here is the file
You need to adjust the position of the 'glass part' from here: Code: ["beaconlight_red", ["beaconlight"], [], {"pos":{"x":0.40414, "y":0.25501, "z":1.31983}, "rot":{"x":0, "y":0, "z":0}}], You also don't have any Beams that makes the beacon attach to the car, so it's basically sliding down the roof.
Look at the file, there is a 'beams' section. You have to define a beam that goes from a node of the beacon to the a node of the car, so that you attach the two parts together. Press K to view the skeleton. Press L to view the node names and etc. http://wiki.beamng.com/JBeam_Reference#beams