Just so you know which wire is which, on the Venture, both generations, Yellow is Brake, Blue is Running Lights, Green is Right Turn Signal, and Chocolate (Dark) Brown (as opposed to the lighter Brown which is a hot line with the bike on) is Left Turn Signal. Of course Black is Ground. Perhaps one of the easiest places to get to all of the wires is that 13 pin connector Steve is talking about. It is located under the seat behind the gas tank about in the middle of the rear fender area. It has all the wires for the rear lights going through it as well as all the wires for the CLASS system, the trunk light, etc. It does not have the lighter brown ignition wire so you can't get confused.
The converter Heather pointed out will work fine, but myself I prefer to use the more expensive power isolator type converters which have a separate 6th line on the input that goes to battery power. The great advantage of these types is if your trailer develops a short in the wiring, which sometimes does happen, it will not take out your bike's lights or blow a fuse. It has circuitry inside it that uses the power coming from that extra red wire on the input to power the trailer lighting and not the individual circuits for brake, turn, etc to power the lights. Worst thing to happen is the fuse that you put on the red line pops instead. Hoppy makes a good one of those, they run about $40...