Hello folks. What a pain this has proven to be!
TLDR: I just replaced the whole locking mechanism
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This being said, I did remove the locking mechanism and clean it first but in doing so a spring fell out and it was near impossible to replace and I ended up pinging it across the garage... hence the new lock. There are some really useful videos online that go through the steps and you can pretty easily get to the lock with a little bit of fiddling.
The main issue on my vehicle, the Mk2 Estate, is getting the metal plate/cover off from the inside of the door panel that allows you to access the window glass and locking mechanism. It was held in place with pop rivets, now luckily for me I had dealt with this previously by drilling them out and drilling in holes for screws. The plate screws directly into the window regulators guide rails (forgot the word for them now 🤣). I've found the best way to put this back together is by lowering the glass which pushes the guide rails forward enough for you to screw it in from the front and apply a nut to the back.
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My best, rough, guide:
Lower the glass, I lowered it all the way at first and messed around but you could probably lower it just some of the way and hold it up with tape
Remove the interior door panel
Pop out the three plastic covers on the side of the door (in all honesty I don't know what the bigger one is for)
Unscrew the top hole, which holds in place the door lock cylinder
Pull on the door handle and wiggle out the door lock cylinder (the bit that you put your key in)
On the side of the door handle there will be a cable that goes to the lock, detach this with a flathead screwdriver
Here the memory gets a little foggy, but unscrew the bottom hole which should help to release the door lock
There could be some connectors around here that need moving with a flathead screwdriver but IIRC it should start to just wiggle out
You should now, probably, have the door lock in your hand
Now, if you want to clean out the microswitch and try it yourself I would recommend that you go and watch this video, there's also some other videos on YouTube that are pretty good.
In doing this, please, please, please be careful not dislodge any of the springs around the locking switch bit.. if you do you will either need some serious DIY skills and patience that I don't have or you'll have to buy a new locking mechanism. For the amount of time (and times I stabbed myself with a flathead) I decided it wasn't worth trying to get the spring back in (or looking for the spring I flew across the room) and bought a new one.
Fitting a new one is pretty simple, basically just get it in place and line up the latch with the hole in the door and screw it in. It is worth noting here that my new mechanism did not have either of the cables attached to it. I installed the interior cable, but forgot about the exterior cable and then had to uninstall the unit to go and get the cable from the old locking mechanism! Don't make the same mistake as me lol.
Once it's in you can give the connector a spray with WD-40 (the electrical connector one) and plug it in. Make sure to connect the exterior door to lock cable, then you can pull on the handle to pop the locking cylinder back in and screw it in. Next up is fitting the metal cover again, you can see above that I used screws for this and how I got it back together.. one thing to make sure of though: the interior cable needs to come through the right hole ... I might've put it through the wrong hole and fastened everything up, you can imagine the door didn't shut because the cable was being pulled too tight.
Anywho, after all of this shenanigans it works. If it only it was as simple as writing out this post. It's not the easiest and it gave me about a week of pure pain but now I can happily say that when I leave the headlights on and open my door the car tells me off 😂
I did get a couple of pictures, but I'm not sure how they really fit in to what I'm saying so I'll leave them out but if anyone would like to see them let me know.
Good luck!
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Resources:
Microswitch Cleaning Video
Microswitch Replacement Video (this has two parts)
Door Lock Mechanism Replacement Guide
There's plenty of more videos on YouTube but these were the most helpful.