I also installed as an addon and followed your directions. THe vehicle does not show up when selecting a car for duty in the garage. I edited the dlclis.xml as you stated to add the line <Item>dlcpacks:/xmastree21tahoe/</Item>
Nothing shows up.
I think at this point I need to just manually edit the handling for fbi2 with he values you have in the addon file under handling to ensure it doesn't always flip.
dlclist.xml
I have installed the vehicle as replacing the fbi2 one. However, it rolls over way too easily. I changed the handling to "granger ing the vehicvcal.meta file but it still rolls over like a dog wanting its belly pet.
How can i fix this?