Amazon! Good luck, I'm assuming it's the same in the US as it is in the UK; they don't want to give you the codes if you got the Collector's Edition.
Yeah, that's right: The customers who pay less get the Stealth cars, you don't. I had to basically fight them to get my codes...
First of all, I emailed casually asking if I was going to get a code, and they said yes, by the end of the 25th, if I didn't have a code I should email back. I did, and they said I wouldn't get a code, so I emailed back again to ask why, and they said if I don't have a code by the 27th, email back again. So I did, and I was told no codes again, so I was like fair enough, two out of four reps got it wrong, I can live with that, but I requested they announce the fact that the Collector's Edition wouldn't get codes.
Then I checked the Stealth Cars discussion on Amazon.co.uk and found that one person had been given codes, so I emailed Amazon again telling them that and asking for the codes; they said "sorry, you're not getting codes." I emailed back saying that's not fair, but they just said "sorry, no codes."
I checked the discussion again and someone else had said they got codes, and another said they got £5 off their next order with Amazon as an apology, which obviously irritated me a bit... I started a new email enquiry instead of replying to the ones they'd previously sent, saying I was angry that they'd treated me like I'm stupid or second-class, and that I wouldn't stop emailing them until I had a 'satisfactory outcome to this issue', meaning either the cars or £5 for not actually doing anything other than sending emails.
Within 5 minutes I had the codes. I suggest you email them a lot and see what happens, only three people on the Amazon UK site seem to have gotten the codes, others haven't had any luck at all.