Seeing many many grey cars out there sucks, and its because most peoples internet upload is pretty much shit. i was looking around at various tutorials for something completely different when i came across this page. It basically shows you how to create a file that trackmania interprets as a download link to the files that most people would just be uploading from their computer.
basically it makes your skin or custom item get downloaded very fast.
http://www.trackmania-carpark.com/faq.php its in that faq and many other places, but this tut explained it the best for me. i used dropbox.com but i'm sure there are other places you can use just as long as it is a direct download link.