Assuming that every thing is working as it should, and the switch has no more current going through it than citroen intended, can you work out precisely which bit is generating the heat, there must be a high resistance point in it somewhere. It's most likely the contacts, but could be a bad connection either from one of the wires or something internal.

This might be no help but i burnt my old loom by bridging the wrong things
Now sporting a non dim dip era loom with relays to take the load of the switch for the halogen bulbs and just upgraded the wire feed to headlights