You can win auctions by either bids or the far easier method of buying out.
Head to any festival and use
to go to the auction house and use the search to limit your search to a max buyout of anywhere from 20,000Cr (didn't find many cheaper than that. Highlight the one you want and press
and select 'Buy Out'. Once complete return to the main auction house tab and go to 'My Bids' and again press
on the car and select collect car to get the car and the challenge.