Refer to the link on how to install a relay to take the high amp load off that starter/headlight circuit. That seems to be the major cause of eventual failure of the little slider switch contacts. When I did it on mine, I simply mounted the relay inside the headlight bucket, bolted it down with one of the three bucket mounting bolts. At the time I first did the change I figured out the difference in amperage but it seems like it dropped the amps carried by the starter button contacts from around 3-4 amps to less than 1. Someone can correct me on that, just going by memory but it is a substantial drop.
http://www.rattlebars.com/mtz/starter.html