You said they're addons but you're replacing the default vehicles...so which is it?
If you are installing these packs via dlc packs, ensure that you are adding the names of the packs to the dlclist.xml file as outlined in the installation instructions, in the correct place (at the bottom of the file) and the correct format (using backslashes or forward slashes) normally it will give you the proper format, saying something like "Step 2: Add the following line to the file 'dlclist.xml', found here: mods/update/update.rpf/common/data/dlclist.xml (ensure edit mode is on and right click on the file and click 'Edit', then add the following line to the bottom, just above </Paths>:" (then the instructions will give you the exact line you just have to copy and paste to the aforementioned location, something like, <Item>dlcpacks:/bcsoaddon/</Item>.