Ads meant nowt - in contract law they are an "invitation to treat" as opposed to an "offer to sell" iirc. Mistakes can and do happen.
Why didn't you check the car over fully before buying it?
Invitation to Treat is only up until cash is handed over.
Once cash is handed over its a contract.
If the above mentioned contract was based on a piece of information being untrue (Heated Seats), then thats false advertising.
Your right though, if the OP didn't hand over money he wouldn't have a leg to stand on, because it would only be an 'Invitation to treat'
Similarly if the car was priced at 100 euro instead of 10000 euro, the OP couldn't walk in and demand to buy it at that price because its only an 'Invitation to Treat'
The AD only provides proof of what the Garage advertised. In itself its worth nothing, but when combined with the OPs word against the Garage it backs the OP up.