Boot lid problem

I had this problem when we first got the car.
Your right, the sensor is intergrated into the locking mech of the car and you will need to buy a new lock.
They are quite cheap.
The reason it goes is because the liquid from the washer bottle drip down onto the lock and in time the sensor fails.
The cure we found was never to use the rear washer but just the wiper.
I blocked the end of the pipe off further down with a bolt stuck down the hole.
As for refitting a new lock this I found tricky.
You need to remove the interior panel on the hatch.
Then atttcah new lock.
Climb into the boot with rear seats down and get someone to close the hatch.
That way you can position the lock, so when shut push the lock upwards so it is just holding the ring, so that you get a secure lock then climb out and unscrew the rubber bumpers either side so there is no lateral movement on the boot lid.
If not done the lid tends to bounce on the lock.

You need to remove that big black panel which just pops on.
The sensor mech is inside the locking plate which is held on with a couple of screws
Unfortunatley you need to replace the whole thing as the locking plate is a sealed unit.

Ditto - my rear washer hose kept popping off every few months or so and each time I would try and fix it a bit better to make sure it lasted longer. Eventually it went all together and I ended up repairing it by attaching some old rubber fuel pipe from my karting days between the motor connector and the plastic hose line built into the boot structure. Many high quality pull ties were used to secure it on as tightly as possible and since then it's not broken or popped off since so it is possible to repair it if you ever want to get the rear washer function back.

I had the same problem but before I bought a new lock, I sprayed the lock mech with penetration spray for seized blots etc, that worked a treat on mine and had no problems since, might be worth a try before buying a new lock if you haven't already.