Nice thing about the Liberty is that there's plenty of room in the housing to add another bulb. The guy that installed mine (Randy @ Spillar Hitch, Austin) told me that Jeeps makes models for Europe that have an extra bulb (not sure what for) and they use basically the same housing, just without the extra socket. So there's a spot in the housing tailor made to add a socket.
He's done so many of these that he doesn't even buy the Blue Ox kits anymore. He just buys wire, connectors, and sockets in bulk and wires them up like the kit would.
2008 King Aire 4562, Spartan K3(GT) w/ Cummins ISX, Motosat HD-SL5 w/ SWM-8, Multiswitch, HR34 Genie
2014 Jeep Grand Cherokee Overland 5.7L V8 Hemi w/ Blue Ox Aventa LX Tow Bar and baseplate, SMI Air Force One brake