Just for giggles. I did a Rip/Replace of the aircraft. On first run, the default MD-82 was fine, the default MD-83 was missing the external model and then the default MD-88 was fine. Without restarting the sim, I loaded the MD-83 again after having just loaded the MD-88 and this time it was fine. So, even though I'd done this before and it didn't work then, it did this time.
Here are the steps for livery additions if you are using add-on linker like I am:
1. Enable the skins in Add-on Linker
2. Open the Fly the MaddogX Load Manager.
3. Within the load manager, select the aircraft you have added skins you enabled in Add-on Linker and then click the Liver Manager Button. You should see all your liveries in the Added column.
4. Click the Apply button in the liveries manager.
5. Open GSX and enable the MaddogX
That should work. It's working on my end, although I think I'm running into a skin number limitation for the MD-82 in the load manager. It appears it doesn't like more that 332 skins for some reason. Still trying to figure that one out, but for this model issue, the recommended rip/replace has worked and the steps above allow for the seated passengers.
Sorry I didn't get a chance to post this earlier.