Do you mean that the factory plug on the vehicle is round? Probably 7-prong? If so, all you need is an adapter. They are available at just about any auto parts store. I've seen them at Kragens, PepBoys, even Wal-Mart. Should look like this:
http://www.etrailer.com/Merchant2/gr.../ele/30305.jpg
Plugs into the vehicle plug and is available in a variety of configurations on the other end to match your trailer plug(i.e. 4-wire flat)