My mk3 has a fairly new battery (2 months old). I didn’t drive him for about a week when it was sub 0 and then he wouldn’t turn on - turns on boot had popped open. This has happened a few times even when driving before and I thought was because of my boot rack which is usually fixed by tightening the boot rack on better every so often. However even after this and a jump start the battery died the next day again. I tried it again this morning and when I drove a few lights were on (petrol light) even though I thought I had more petrol and the dials seemed a bit jumpy, plus the boot opened when driving again. I am now thinking electrical issue, but I’m very reluctant to go to the electrical specialist to diagnose it because I have not many monies.
Are there any obvious solutions I should try before taking it in?
Thank you
Is your passenger footwell damp/wet?
Leaks into there are common and there are sensitive electrics in the path of the water that gets in.
The boot light can and probably should be switched off.
Even a new battery will die or be severely damaged if left flat for any length of time.
it’s not wet/damp on passanger footwell. But I looked at the drain holes - they are blocked, could this be the culprit is water in them? I am probably going to have to unblock from underneath.
It’s not the boot light but the actual boot that is opening!!!
Just thinking of your battery drain.
If the boot opens and the light is not switched off, it will drain your battery.
If the roof drains are blocked and you get a heavy down pour, I wonder if you could be getting water in the electrics for the folding roof.(r/h side)
I test drove a car that kept popping the boot open and it was down to the electrics having been soaked with over enthusiastic applicstion of a pressure washer
I think this is the problem. am I being too optimistic in hoping that unclogging, syringing, and a hairdryer is going to fix this?
After fitting new battery or recharging a totally flat battery on a NC you may need to reset the DSC (Dynamic Stability Control)
- turn key to on position, but not starting the engine
- turn steering wheel all way to the left until it stops (difficult without power steering)
- turn the steering wheel all the way to the right until it stops
- Re-center the wheel
Hopefully not.
Drying out cured the car I drove. 
Don’t clean the hood drains from under the car! Go in from the top with your trombone cleaner, plenty of how to explanations around including video.
The flickering dials and odd instrument lights could still be water ingress, check that small fuse box is dry inside the passenger side at bottom of the A pillar. You could also check the tightness of both battery terminals, this can cause problems if one or both are dirty or loose.
If water is found inside, would usually come into the passenger foot well and as said down into that fuse box if unlucky then you need to fix/seal your scuttle grommets. I’d be checking them anyway and sealing them if not already done. Again another search brings this subject up, if you can’t find anything or unsure shout up.
Are you sure you looked at the drain holes and they were blocked?
My tromobone brush isn’t going through the drain holes there is something in there!! I think I need a coat hanger and to go through the bottom I can see water in there though so I think it’s the culprit. I’ve checked fuse box and it looks ok there is no water actually inside the carbut he’s been misty and now I know why. As far as I can see its just the drain holes are clogged and very much hoping that clearing the blockage and some drying out will be the cure, it looks like it is the culprit. At least I hope it’s the culprit and not anything bigger but will also check battery terminals thanks
yes. My trombone brush is literally getting stuck and not going through and I can see some water in there 
It’s always best tackled from underneath. That’s usually where the blockage is, up against the rubber flap in there. You can see exactly what your doing, clear it out and make sure the flap is sat back properly.
Dicking around from 3 feet away with some stupid brush causes more issues than it solves.
The only downside is getting under there in the first place.
To find the bottom end of the drain tubes, lie on your back parallel to the car with head by back wheel.
Reach straight under just in front of rear wheel with palm upwards and bend wrist to contact vertical panel facing away from you. Feel around a bit and you should find the plastic tube which stands just proud of the vertical panel
I had the flickering lights, jumpy dials after Id had battery on charge but no issue with boot lid.
Turns out my positive connector clamp does not fully tighten on battery terminal and I can actually pull it off even when fully tightened , just a thought ?