I'm sorry, I don't know where X1 is in a 140. Looking at a picture of the switch it looks like it has a cable and plug that would reach behind the dash somewhere.
As far as using a switch or a relay, they are the same thing except one is intergized manually and the other electrically. The actual contacts that close are the same. If the switch can be designed for the amperage needed then only one thing in needed to turn on the headlights (in this case, switch low to high). It becomes a cost factor.

Paul

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84 500 SEL (307,xxx miles)