Well, I found what looked like a Russian site that appeared to be the answer to my dreams-I really desperately needed a black Mercedes car. I also dled a few other things, all of them civ vehicles.
I use Mozilla, and it doesn't let me dl them anywhere other than my desktop, so that's where they went. I then took all the files and chucked them in the Addon folder. I started up the game, and nothing. So I tried putting them in the Res\Addon folder, and still no luck. Then, I noticed that the file extension was .rar-and I've never seen this before. So I tried changing it to .pbo and nothing (hey, it was worth a shot).
These addons had just one file apiece-nothing extra, just one solitary file.
Any ideas?
edit: I have also considered that maybe these files are only for a higher version (I am running 1.85, I can't upgrade due to some corrupt files or something), but they are from two years ago so I doubt it.
re-edit: actually, they're from 6/6/04. Was something higher than 1.85 out then? I think it was... still doesn't solve what .rar is...