Hi,

I run a car site that gets a lot of traffic from search engines (over 160,000 uniques and 560,000 pageviews per month).

I am looking to start selling media to automotive companies, any ideas on where to start?

Yes find a advertising company who will do all the work.

What kind of cars your company is manufacturing. Sports car luxury cars, business cars all have different method to advertise. You have to target your oriented customer reach about geography possibility income of your target customers will be they able to purchase it or not and many more factor you have to clear. After this you have to SWOT analysis. In SWOT you have to find out below thing for your product.
Strength
Weakness
Opportunity
Threat
Apart from this you can hire an advertisement agency that will do all above task for you.

you should consult a proffessional advertisement company for this task...