Rebecah made one (it was on her previous site), but it moves like the radio car, so you can't actually control it. It just drives wherever it wants. And backwards, if I remember correctly.
There is also one modified from a car at MTS, but it's big and lumpy, and not very useful.
It seems to be deleted, however. You can see it here:
http://thesims-denisa.blog.cz/rubrika/nabytek-hacky-dekorace/3Other than that I don't think there are any.
There isn't a good way to make a movable wheelchair, at least one you can control. There are a lot of chair versions, however.