You will need 4 diodes total. One for each tail light and one for each brake light. This is how you hook it up:
http://i.imgur.com/Gc4pRwQ.png
The reason for the three connections on the specialty diodes is to make it easier to hook up. Bear in mind that those specialty diodes are $10+ each ($40 total) and Radio Shack sells 3 amp rectifiers for $1.35 a pair.
You could use another diode per connection to keep the car's voltage from going back up the trailer harness when you are driving like this:
http://i.imgur.com/w9YwNlY.png
I just cap off the connector that's under my hood. Oh, and if you run the harness up under the car on the passenger side, there's an existing cable run that you can fish the harness alongside and then go into the drain hole just forward of the muffler. You'll have to cut a slit in the plug to pass the harness through.
RK