I can't help with the switch but if the old one melted you need to check the column switch as well and all the wiring inbetween!
Odds are the column switch has also melted but is just keeping the contacts together - the result is more current than either switch can cope with. It is a downward spiral too until something breaks. You could put a new switch in and get away with it but you may find the wiring is damaged too in which case a replacement switch may not last long either.
Worst case if you can't get a replacement it is possible to rig up something temporary to separate out the different circuits (headlamps up, side lights, head lights). it won't be pretty but it will get it working again until you can find a replacement.