Why is it wrong to include "neked"?
Why is autója also ok?
Because that would be the formal you, which is conjugated like the third person.
"Miből van az (Ön) autója?"
Can I use "Mit a te autod késült?"
Even with készül (which is the intransitive version of készít. This language is so neat!), it would still need to be miből. Készül doesn't make sense with a direct object.
Also remember the word order: verb directly after the question word.
Ack, right. Word order. I keep making it more fluid than I should.