MM2X Forum

I'm wondering if there's a mod or some text that I an add to change the ai cars to ones I've downloaded for mm2???

This could help you:
https://forum.mm2c.com/viewtopic.php?p=292950
Many of the pictures in that thread are not working because the thread is a bit old but just read the instructions and I think you'll understand it

BTW although it says modelling your own cars and using them as traffic, I think you can also use this tutorial to get already downloaded cars converted/modelled by other MM2 modders to work. It may not work if the downloaded vehicle is very high in polygon count though

The only AI cars that you can replace with download add-on player cars are the city cops.
The edit has to be done to the .aimap and .aimap_p files, replacing the parts where's written the "vpcop" basename with the basename/s of the car/s that you want to use as city cops on the places of the default Ford Mustang GT Cruiser.
You can easily get those files by editing the ones present in the patched of this add-on car. Open the .ar file of one of the patches (it's one for each city) in the archive with WinRar, and extract and edit the files in them with Notepad. At the end drag and drop the edited file in the .ar file kept opened.

If you want to replace the traffic cars, the operation is a lot longer and it's very similar to the one of making a new add-on car for the game.