I found directions and components in an MOA magazine from the late 80's or early 90's.
Use a quilted insulated shirt (you need layers). Fleece won't do unless you put an extra quilted layer inside it. You need something to hold the 'lacing' of the wire.
Get some 26 gauge wire, solder the end to form a needle, and weave it into the insulated layer of the shirt. Be sure to use LOTS of wire, and don't let them touch each other (1/2" apart is too close). Start and end at about the same place so you can solder a cigarette plug . The heat the wire puts out is a function of the length of the wire. To do sleeves, I would start in the middle bottom of the back, go up the middle, down one sleeve in a looping fashion (down and up), then across the back, up and down the other sleeve, then back to the start.
Start the bike, plug it in, and get warm. It will hardly draw any amps.
It's relatively simple, it works, and you'll wonder why you didn't do this a long time ago!!!
I've done this to an old insulated ski vest, an insulated shirt (x3).