@ohno_san I don't think you can have automobiles in the Merchant Account. We have our retargeting inventory in our Business Data in the Shared Library of Google Adwords.
I think your gonna like the Business Data in the Shared Library @ohno_san. I used Google Merchant Center a few years back. You have to make sure when you load the inventory in Merchant Center, you have to make sure you have "Publish to Google Shopping" unchecked. It's automatically checked off to send into shopping by default. Then I simply connected the Adwords account and the Merchant Center Account together. The hard part was identifying the specific vehicle the visitor looked at. To accomplish that, I used jQuery to identify the Vin on the page and pushed the Vin number back to Google Adwords as the Product Id. That was 3 years ago though!