The warmer switch might be bad, or perhaps a mouse chewed some wires in the warmer circuit.
You can test each warmer individually by disconnecting them from the harness (under the handlbar cover/foam). Just do one at a time and remember/mark the wires if you are worried about forgetting how to plug back together. When you have the first warmer disconnected down to just the pair of leads for that warmer, hook both leads to an automotive or power equipment battery charger or 12V battery, again be careful and be ready to disconnect if you suspect something is going wrong, I've never had any issues on mine, whether the warmer is good or bad. If the warmer is good, it will heat up in a short amount of time.
If you have a multi meter and don't want to try the method above, you could still disconnect the warmer as above and test each warmer individually for continuity between the pair of wires. If no continuity, the warmer is bad. If you have continuity, the warmer is likely good, and there is also an ohms resistence value but I don't know what those are offhand.
To be thorough you could also test each wire lead one at a time to "ground" for continuity, with ground being the metal handlebars, and you may need to find a bare/rubbed area on the bars to make good contact, as most Yamaha bars have a coating on them that the multi meter may not be able to read thru. If you get any continuity from a wire to the bars, that warmer is shorted and may not work properly or at all.
If you run the tests above and find that you should have good individual warmers, then you have a wiring issue, switch issue, or possibly stator/magneto issue. I don't think there is anything else in the warmer circuit on a 1994, but I could be wrong.
