How do you make the footwell lights come on when you turn sidelights/headlights on?? I'm a complete novice when it comes to this sort of thing so I need easy instructions please. 

Question..

Why would you want to?

Itd be extremely annoying after a while & off putting whilst driving

It would also require rewiring of the cars existing wiring loom which will be a pain for resale, unless you add an additional set of lights and wire it to the loom at the back of the light switch

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On my car the footwell lights are always on as standard and linked to the dimmer controls.

Sent from my SM-G930F

They are only a dim red light on the Focus ST which is okay to be on all the time.

I wouldn't want bright halogen bulbs lit up all the time, that would be distracting.

Too true.

They are very subtle even on maximum brightness which is the way they should be.