My favorite project.
The light is opened by sliding a blade under the plastic piece and gently prying out. Do catch the glass lens before it falls to the floor and breaks into >1,000 pieces. The problem is that the electrical contacts inside the switch do not make the best contact and with a little bit of corrosion, make no contact at all.
You will need a fine needle nose plier to remove the retaining nut and remove the switch. Once that is removed, I won't even attempt to tell you how to open the switch and remove the internal contacts. Just stare at it until you have each and every microscopic detail memorized and very slowly proceed, or better still, photo each and every step close up as you open the switch. Once you remove the contacts, gently sand both sides with 600 emery (something fine), wipe them clean them and coat with a light grease like Super Lube. Re-assemble exactly as they came apart and the light will work for years. I've done all mine and have not had any further problems.
The first time you do this is a complete bugger but afterwards, not too bad actually.
