A volt meter or test light would come in real handy right now.
I tell everyone, every chance I get that every one should have one or both of these.
A reasonable digital multi-meter can be purchased for around $20. Like this one:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=37772 A test light is a simple device that sort of looks like a screw driver with a wire coming out of the back and a light bulb in the handle. Clip the wire to ground then push the point into a circuit that you think should be on. If the light bulb lights, you have power. Here is an example:
http://www.stu-offroad.com/electrical/testlight/testlight.htm The test light is great because it doesn't need batteries, is more rugged than a mult-meter (it will survive getting wet) and is very easy to use. The down side is that it give a "pass/fail" result, you can't use it to tell how good that battery is.
If you have a test light, clip the wire to the frame of the car (finding a good ground on a car is getting hard these days!) then poke the tip into the trailer light connector. Note that the bare connector on the 4/5 pin flat trailer connector (car end) should be ground (if it isn't, that might be your problem!)
Test a circuit that does work (like the turn signal) to make sure that the test light is working and you have clipped to a good ground. This will divide the problem in half: if you have tail lights at the connector then the problem is in the trailer, if you don't the problem is in the car.
If you have a multi-meter test continuity from the trailer connector to ground. The odd pin on the connector (the covered one on the trailer side, or bare one on the car side) should be ground. If you are looking at the connector, and you see that the ground wire doesn't go anywhere (which is actually fairly common) fix that first. Many times the trailer wiring tries to use the hitch as a ground return to the car, but that is very unreliable.
If you have a ground connection then test each of the other connector pins for continuity. Use the "x1" scale on the "ohms" setting (often signified by the Greek "omega" character). For all practical purposes you should read very close to zero ohms. (1 ohm is pretty close to zero!).
If the tail light circuit shows open then either all the bulbs are burned out (unlikely) or there is a wire broken somewhere.
If the car tests out okay and the trailer tests out okay then check the ground between them again.
Word! 
Linear Mode
