Marcus ,
I had a similar problem many years ago , just after the first restoration of my 400 . I fitted a new wiring loom and later found that all the screws at the back of the ignition switch needed retightening.
In the past 25 years I have not had a further problem except that when I have the head lights on the ignition key gets warm , probably caused by worn contacts within the switch. I suggest 400 owners fit relays to both the high & low beam circuits and the main power will then not go through the light switch.
I'm about to fit a new loom to my other 400 and will solder the ends of the wires before fitting them to the ignition/light switch, this should give a better contact .
I doubt that you would achieve much by pulling the switch apart except cleaning the contacts , I think your problem is loose wires .
The switch bracket is held onto the steel dash frame with a few screws , the central wooden dash panel must be removed to access them .
depending on the length of wire available behind the switch its possible to draw the switch away from the frame and tighten the grub screws that are fixing the wires to the back of the switch. But more than likely there will not be enough length in those wires , so I suggest you drop the radio and you then have reasonable access but poor visibility.
I have never pulled one of this switches apart but have a spare which is probably no better than yours .
Good luck
Geoff
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