I had the same problem. You have to change the model/texture file names, and the names of the vehicle in the meta files so it's an add-on and not a replace. For instance, I wanted to change the handling and the audio hash for the charger, which is police2. I renamed it to pd2, and copy-pasted in a handling for it. You'll also need to copy-paste a carcol for it, or the lights won't work.
Anyone know how to fix the dirt textures? I love this pack, but there are a few with broken dirt maps.