Replace 7 pin trailer connector with 13 pin (Fabia I combi 6Y5)

Hi.

The installed 7-pin trailer connector on my Fabia I combi (6Y5) is very flaky, and my plan is to replace it with a new 13-pin connector, which most trailers have now and not need to to use the bulky 7- to 13-pin adapter

However when the old connector was removed i noticed there was an extra cable going to a little switch which is grounded (pin 3) when a trailer is not connected (i guess).

What is this extra cable used for? Is it possible to just rewire the normal 7-pin to a 13-pin connector?

I do not need constant power or ignition switched power to the trailer.