NOTICE FOR THE BETA4 UPDATE: THIS METHOD WILL NO LONGER WORK WITH CARS FROM EARLIER VERSIONS OF TRACKMANIA AS TRACKMANIA2 NO LONGER SUPPORTS THE OLD STYLE CAR GEOMETRY!

You can actually have custom cars in Trackmania 2. I was able to get my Mazda RX8 in-game. Proof:
http://cloud.steampowered.com/ugc/594694275296162039/4D9878EBB341348B8A271824934F9BA9975A16AA/

Here's how:

1. Add your car(s) to My Documents\ManiaPlanet\skins\vehicles\CanyonCar (Documents\ManiaPlanet\skins\vehicles\CanyonCar on Vista)
2. In TM2, goto Editors -> Manage Packages
3. Create a package with one car in it. You may need your validation code.
4. (if multiple cars are to be added) You will need to exit TM2 before creating another package (likely a glitch that will be ironed out soon)
5. Select your car in your account settings as usual. You will now be driving your custom car instead of the standard Canyon car.

Note to Moderators: Feel free to move this to where it should be if this isn't the right forum

I think this belongs in the tutorials forum.

I was adding a note for that just as it was being moved. Thanks.

k,cool..now when you make it, it turns into example: stadium.Skin.Pack.Gbx

now even if you have the .loc made for the original skin, do you have to upload say to filden now..

stadium.Skin.Pack.Gbx ... uploaded to filden
stadium.Skin.Pack.Gbx.loc

like that.. ?? so others can see it online

It should already be available. If your custom car is online then along with the .zip file I guess you should add the .loc file to the package? I only 100% know how to get the cars in-game locally and P2P, and I've never really messed around with .loc files and packages yet. I'll have to experiment with it and see what happens.

I don't use .loc files much for custom skins, since I always enable p2p.

Do I need to make a new .loc file or is the .loc for the original car enough?

I'd assume that the original .loc is enough.

What about the damage? Does the custom car get damaged like the orginal?

car needs to have special "damage file" -> old cars won't have damage (in case they won't be updated).

tuto in french here with pics
http://forum.maniaplanet.com/viewtopic.php?f=20&t=3920&sid=3a2b01efc6adb6df20d5b46814a2c5d0