Hi 190 D,
Do the knobs heat up when you have the lights on? If so the illumination bulbs could have been replaced by ones with too high a wattage. I think they should be no more than 0.4 watts. On my 300E the bulbs are tiny push-in types with frosted glass. I had replaced a heater knob illumination bulb with an apprently identical one but it glowed brighter than all the others! I can't remember if it heated up the knob too much but I replaced it with a genuine MB one because it looked odd. I have only found these at MB dealers but they are cheap (£1 each I think). the part number for 124s is 000 825 00 99, the same for 190s? Are the red "hoses" you mention the bulb illumination wires? These may also get excessively hot with incorrect bulbs. If it isn't the bulbs then I hope you find out what it is.