Invisible vehicles or missing parts are usually a problem in your video settings and/or you FSX.CFG, usually some tweak you shouldn't have applied.
You might try all all these:
- Reset your video driver to default
- If that doesn't work, try to delete your FSX.CFG, so a new one will be created with all default settings. Apply only the HIGHMEMFIX = 1 tweak and nothing else, if you use FSX.
- If that doesn't work, be sure you are not running the sim or any of the addons that starts with it in Compatibility mode.
After resetting the FSX.CFG to default, you must reinstall GSX, because the installer will have to add the paths to the GSX Simobjects folder, or else it won't work.