Now so far we do not know if it really is the switch that is the problem, but if it is there is no need to fit a relay. One can simply solve the problem by fitting LEDs in all three stop lights. They draw so little current that there is not really any impact on the switch. There are LED bulbs that combine stop light/driving light and of course also stop light bulbs only. Just remeber to get red ones, not white. A white bulb behind a red glass renders a pink light. I have done this a few years ago, no more switch failures.
