you have to complete certain things to unlock stuff. this website shows you how to unlock everything, including karts.
http://www.videogamesblogger.com/2008/04/18/how-to-unlock-all-mario-kart-wii-secret-characters-bikes-karts-and-more-guide.htm
you have to have gold in all the cups to unlock it :)
keep playing
you have to do time trial races to unlock characters/vehicles
to unlock dry bowser, you need to get a one star rank or better in all 150cc Wii cups (all the cups on the top)
to unlock everyone you have to play grand prix all classes and to unlock the rest you must do time trials it takes a while but its fun
win 150cc to unlock
all you have to do is get a gold medal in all the other races of all the other cups, then you should unlock mirror.
Unlocking karts is really simple! To unlock, win a series of all the cups on 50cc and you may even unlock some characters along the way. Also, you can go on different cc's and race and get stars on all of them and you'd unlock them for sure. Also, on time triles, get very good times and unlock fast staff ghosts for karts.
i think you must beat 50cc with all gold if you do this you will also get King Boo
get 1st place in all the other races and i would recommend Yoshi to race with (just to unlock stuff though)
To get mirror cups in Mariokart Wii you willk have to complete all of the cups on all engune sizes
There is only 1 to unlock and that is Toadette the pink toad. To unlock her you have to at least do all 32 courses in time trails (but you dont have to win them)