Did you check the headlight to make sure it is a working unit?

I know like I said dumb...

My 240 has had all kinds of problems with the contact made between bulbs and holders and the fuse holders etc... I cleaned every contact I could find onthe car but still had to gerry rig one bulb in the tailights to make it work consitantly.

The heat thing would indicate to me a short somewhere in the system. You can test it at the fuse block with a ohm meter to see if the positive lead is shorted to the body somewhere. Make sure that the correct amperage fuses are installed. This may have had a short before and the PO put in an oversize fuse to compensate for it blowing fuses. This is all too common unfortuantely...

Take your time and I'm sure the solution will appear if you track the problem logically. Luckily this is probably not an under dash issue. What a PITA that stuff is to test while you lay on your back.

The blown fuse on the fogs is probably due ot the fact that the two fogs are wired together so when you turned them on the bad wiring on the non-working headlight side shorted and blew the fuse. Anyway, the wiring on the passenger side sounds like the problem. Probably where it goes through a body part i.e. where it is tied to a frame piece or travels through a hole in a frame piece.
You get the picture.

Good luck.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique