Installing Harley Davidson Chrome & Rubber Hand Grips Large 56263-96A

The harley heated handgrips are notorious for failing.  Some people have reported finding a loose connection on the ground wire, like they did not crimp the terminal on the end of the wire at the factory.  If they are wired externally of the bars (almost impossible to wire those any other way) then it should be easy to follow the wire down to inside the headlight bucket or in the nacelle or dash, (depending on what model bike) and check the connections.  If both grips quit at the same time then it's probably wiring.  If one quit first, then it may be the grip itself.
If you need to replace them, I would highly recommend not buying the HD brand anymore, and instead choose Polly grip heaters.  They are made for snowmobiles and have a long list of benefits over the HD type: - fits internally in the bar end, so you can use any grips you want - cost $35!!  compared to $250 for HD - easy to install
I have installed over 7 sets in my friends bikes and not one single failure.