My bike has a HotBodies undertail kit. Previous ownered installed it. For some reason, when I press the brake, the left blinker on the guage comes on. It does not flash, it just turns on and then off when I release the brake. I'm assuming it's a wiring problem. I dont have the instructions, so does anyone have a clue where I should start looking?

It sounds like one of the brake wires is touching or connected to your turn signal wire. So check either the blue or the orange wire (forget which one is L and R)coming from the bike and see if its touching anything else. If not then start disconnecting wires to figure out what goes where.

Does it the blinker turn fully on or does it just slightly turn on? Also, are the bulbs regular or LED's? Creating a ground loop is pretty common. One thing to try is to use separate grounds for the blinkers and the brake wires.