If the bun warmer is too hot then likely there is a short in the heater grid allowing too much current to pass through the coils. The other possibility is that the thermostat is stuck closed, but I've not heard of that problem before. If the bun warmer turns out to be okay, but is simply too hot then an extra layer of *fireproof* foam may help. I don't know if the leather seats in your 264 will have a relay control like the later 700/900 leather seats, but if so then the relay could be faulty as there is temperature and timer control logic in the relay that may have failed.

You need to remove the seat heater coil pads for inspection. Snip the hog rings at the back of the seats, reef out the retaining rods and peel back the upholstery a bit so you can slip out the heater coil grids. The seat (base cushion) heater coil grid will normally be the problem, but if objects have been poking into the grid then damage anywhere is possible. The base cushion is the one with the thermostat button where the wires normally break. With careful soldering or using tiny copper ferrule crimps you can often repair the grids, otherwise seek a used replacement. Note that if your leather seats use a relay control there is no thermostat in the heater grids. The later style fabric heater pads are not repairable. If there is a fried rubber smell then stop using the seat heaters -although rare, burn through holes, melted seat foam and even fires are possible.
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Dave -940's, prev 740/240/140/120 Never overestimate what little I know.