What is the active of these passive?

Please, could anybody tell me what are the active voices of these two passive?
I think they haven’t active, but surely I’m wrong. Maybe there are many passive sentences without the correspondent active ones?
1) I want a letter written to her.
2) I’d like a car painted in red.
Thank you in advance,
Eladio

Thank you pieanne. I've several questions:
1) is "painted in red" as I wrote, incorrect? => painted red?
2) I want a letter written to her. I think that "your active" is really an active one, but do not reflects what I wanted when I wrote "I want a letter written to her", because, maybe I want this letter (written say two years ago) to know what was written to her, and not that someone writes a letter to her.
3) Could you tell me if there could be passive sentences without the correspondent active ones?
And thank you again.
Eladio

Thank you pieanne. No, not "by her" but "to her". Please, consider this:
I'm a policeman, and I want to know what was written TO HER two years ago in order to know if she was threatened or something like this, so I want a letter written to her. Maybe the active is: I want a letter written by someone to her, BUT in this case, except for "by someone", the sentence is the same! And then again, what's the active of this passive? I'd like to write an active for that passive without the past participle "written"...

1) I want a letter written to her. 2) I’d like a car painted in red.

What about?

1) I want someone to write her a letter.
2) I'd like a red painted car.

Ambiguous
I want the letter written to her.

a) I want the letter that someone wrote to her.
=> I want the letter that was written to her.

b) I want someone to write her a letter.
=> I want the letter you write to be written to her.

Maybe it would be better to say "I want the letter that X wrote to her".
If it's THE letter (defined), then it couldn't have been written by anybody (someone, non defined)?