Before we get all enthusiastic about sticking stuff to the windscreen let’s talk wiring.
In a Fiesta the wiring that goes to the roof light and rain sensor terminates at a plug on the left (passenger) A-pillar. The factory glues and tapes this to the headlining so it’s probably better to not try and swap the looms over - instead stripping the three wires for the sensor out from the donor loom and clipping it to the existing connector shell is the smart move.
The sensor wires are mostly “all black” in this particular loom with one distinguished by 3 red dots at the connector. I applied a piece of tape to each that was numbered and match a map I drew out.
Factory twists one pair and leaves the third seperate.
Re-wrapped in cloth loom tape. You can see my creative labels.
The chassis side plug.
Loom pins plugged into the connector and wiring clipped into the cable guides alongside the existing loom.
I fed it into the front edge of the headlining and had it pop out at the mirror location.