Hi!
I have ordered car before official release and played on ultimate pack 4 days before official release, however I do not have codes to redeem PO cars, nor do I have any messages from Xbox live with codes.
Any help?
You should have received a code from your retailer. Either via email or printed on your receipt. Check with your retailer, although it may be too late at this point.
You should have received a code from the retailer or source in which you pre-order the game from.
If you pre-ordered from the Microsoft store, please check the ‘Xbox Live Messages’ section within your messages as this is slightly different to your normal menu.
If you received the game from a retailer, then you may need to check with that store. However, since it’s been a few months after release, the codes may be of a limited amount and they may have ran out. Also check your receipts as sometimes a code can also be printed on that.
Hope this helps
Ordered from miscrosoft store, and I cannot see any messages from them:
And as you can see, pre-ordered in mid September:
Any way for me to redeem those cars? 
Try contacting the Microsoft store
Its only 1 car too
Should be the mclaren 720
As you got it from the Microsoft store, this code might have come in the form of your messages. The Xbox Live messages (so directly from Xbox) are in a slightly different option to where you might get normal messages from your friends.
If you go to the menu > messages > at/near the bottom > “Messages from Xbox Live”
If there isn’t anything there, you may want to contact Microsoft Support.
I got my code and then somehow it disappeared. Not sure if it was a casualty of mass deletion of messages or what.