quote:...  My question is this. When the trailer is connected to my truck is the little battery being charged by my truck.  

Probably not..... If your trailer has one of these installed, it's getting a charge.... http://www.etrailer.com/pc-BAK~20400.htm

quote:.... If so then what wire from the truck is used to charge this battery, and what color is this wire normally?

Use this chart to figure out what wire would be correct for your vehicle/connector. Use a test light or volt meter to search for you correct wire.

http://www.horsetrailerworld.com/home/wiring.asp