Was it here that I spoke about the addon cars? No? Well, then I'll report my findings here as well.
The FEDATA files of the addon cars have some hidden data which makes them accessible only when installed into the SaveData folder. This was obviously done to preserve the integrity of the vanilla game. I do not know what this data is, because NFS Wizard can't read it, but I do know it's there.
I can't pinpoint which of the fields has it but, if you open a FEDATA file of an addon car in NFS Wizard, delete the ID and serial number content and write it again, NFS Wizard will overwrite the data with the values you input, so the problem should be fixed. This will also work for any addon car by third-parties which behaves similarly to the official ones, such as Thomas Egelkraut and jason g trusty's cars.
Addon cars also have specific AI behavior, which NFS Wizard cannot read, but it's there in the Pursuit field of the FEDATA file. NFS Wizard returns a blank field IIRC. Again, you can just switch it from the blank field to "No" and the cars will behave as normal.
I was in the middle of making a barebones expansion pack as a companion to the Modern Patch containing those fixes as well as others (missing colors in the Aston Martin, for instance), but it was necessary to make showcase slides as well as proper vidwalls and I got lazy since Photoshop is not my thing.
I cannot provide audio either since I can't even open the files and I don't sound much like the announcer.