Adding thumb heater to my sled???

My wife has a 90 Indy trail deluxe and I wanted to see if I could some how wire a thumb heater in with no big modifications? I was thinking possibly hardwireing into the grip heaters? Or is that a bad idea?

Also I took her stock grips off so I could powdercoat her handlebars so I had to order some new heater elements from RSI. My setup now has 2 black wires versus the new ones have 3? How should I wire that?

I'm not worried about having seperate switch for my thumb warmer. So I could just hard wire into my existing hand warmer wires?

Also has anyone added passenger hand warmers for these sleds?