Was the truck ever set up as a tractor? If so, there may be a trailer feed but it would probably have separate brake and turn signal wires for the trailer even though the truck would have combined brake light/turn signals. (Maybe that's what hawkeye ran into with his C-60.) If you are planning on pulling a trailer with combined brake/signal lights, then a t-fitting should work or you could splice into the light circuits at the back of the truck. Etrailer.com has great info, as George recommends. If you are hooking up to a big trailer with separate brake/turn signals, they use a round pin connector, rather than a flat pin connector and the wiring is different, as Whitnel says.
Do I have you confused now?