First off, I am not in any way associated with Polly Heaters!
Now for the good stuff. I have Polly's on all my bikes. They are a brass insert that goes into your handlebars at the grip end. Handlebar size is from 7/8th's to 1.5" - they have a adjustable tab. Anyway, to install them you have to take your bars apart but after that life is good; very good. Polly's are made by a small outfit that at first made snow mobile hand grip heaters. They later de-tuned the snow mobile one for use on a motorbike. There are two settings and take my word, on high you better be in 30F or less weather. On medium setting they keep your hands warm, not hot, and as stated before - hands warm, rest happy.
The best part is the kit to my door has been a steady $48.00US; I have ordered so many that I can actually track cost over time.
In use they are better than my BMW GS's ones. Now, they take a bit longer to heat up (use high to get to temp and low for general use) but once to temp they are golden.
After the brass inserts are in the handlebars the electrical part is dead easy. Hot, on my bike through a independent relay that is tied to a switched wire and the battery. Ground, a good ground source; I tend to use the battery ground since it is the most dependable.