There is a place just under the left side of the tank (as you're sitting on the bike) in the little black trim panel. Triumph, of course, sells an accessory outlet. However, you can get an accessory outlet (cigarette lighter) at most any auto parts store for about $5.00 and it should plug right in to the plug located behind the black panel.
There's a plug behind the panel shape like a "T". I picked up an outlet with weather resistant cover from NAPA and it plugged right in. I did have to make the hole a little
bigger, but it didn't take much, and it's worked for a few years now..