If you use fenders and don't do anything stupid like mounting the lamp upside down, the BUMM IQ lamps are probably the best you can buy that are generally available and for a reasonable price, and you just leave them switched on day and night (the LED has a 50k hours MBTF), so no switch required. I always buy the ones with all the bells and whistles including the selfswitching light sensor but I leave them on whenever the bike moves all the same as a safety feature.
I had switched lamps, high and low, on a homemade set of halogens, and the best switch available was presealed with pictails both ends, and a couple of slots in the silicone covering for fitting with tiewraps. However, the pigtails had to be cleanly crimped in and totally covered with hot-melt wire links rather than just wrapped with electricians tape or covered with common shrink-wrap, or water got in. The switch is always the most vulnerable part because it is the only moving part.