Hallelujah!
Only accademic but if you wired your new switch to the blue/yellow and red/orange wires you found under the dash the fogs should come on with side or any headlight selected.
Not clear to me from your description but having eventually found the expected two blue/yellow wires, note they need to be joined together if not already. The reason is only one routes back to the dash, the other being free standing and purely provided to route the single feed to other, n/s of car. You will have to search for far end of it under that wing. A double female bullet connector works well to join wires together if some PO has removed it to add the extension. You should end up with three wires going into it on o/s.
BTW checked under my car this evening and found the unused double female bullet on o/s feeding through to a single n/s. Not easy to check colours in-situ but 99% certain they would be blue/yellow.
