Very happy to hear the mumsnet view on this, as I’m unsure whether it’s the right thing to do. If it needs a diagram I can provide one later 
DM and DF (early 60s/late 50s, so not old!) live in a cul-de-sac in a house they moved into 15 years ago, when I was a teenager. There’s 4 houses at their end of the close, making up a pentagon with entrance to the close being the fifth side. They’ve never had any issues with the neighbours before, they tend to just keep themselves to themselves. I now live 1.5hrs away with DH.
DDad has MS which was diagnosed when I was in my teens and is now quite advanced. They’ve always been ‘grit your teeth and get on with it’ types so while DM is Dad’s carer, she is still working part time. She’s out of the house 10am - 4pm, during which time Dad is generally confined to his motorised armchair in the front room. He passes his days now snoozing and watching the cricket with a thermos of tea, and while it’s a pretty lonely and sad existence he’s learned to cope with his world being small. He can’t go outside without a lot of help and generally he only leaves his armchair once or twice during the day in order to go to the toilet or eat a sandwich mum leaves out for him.
About a year ago the neighbours to one side of them moved and another family moved in. I’ve never spoken to them directly but they seem nice enough - youngish, school age kids. They park their car in front of their garage and that is fine, it’s their land, no issues at all.
The problem has arisen as they have a regular guest who comes over a few times a week (think it is a family member). Our old neighbours used to fit two cars side by side on their drive but new neighbours don’t want to do this, their car’s bigger, they’ve got car seats to fit, totally fair. So they park the guest’s car directly behind theirs, poking out into the middle of the pentagon of the close, which is directly in front of the window into my parents’ front room.
Before anyone asks I don’t know whether they are legally allowed to park there, that’s not really the issue at hand. The problem is that if they park there when the sun is shining, sunlight bounces directly onto the second car and straight into my dad’s eyes sat in his chair. It sounds incredibly petty but it’s really affecting his quality of life.
If my dad were able bodied he could get up and pull the curtain, but then if he was able bodied he probably wouldn’t have to spend 30 hours a week stuck in an armchair 
There’s nowhere else in the house that’s comfortable for him to sit for longer than it takes to have a meal, as he needs the mechanism of the chair to stand up so can’t sit on the sofa.
This has been an issue for a while and there are a lot of areas along the cul de sac that would be fine for this guest to park. However, in typical ‘grit your teeth and get on with it’ fashion they’ve mentioned it in passing but not pushed the issue (which is when the neighbours have said about not wanting to park the cars side by side). Stupidly, this week my dad decided to try and get to the door and speak to the guest who was parking up and ask her to move. As he can’t get outside without help this involved calling to her from the front door and now they have accused my dad of shouting at their guest, which tbf he probably did, albeit only because he couldn’t leave the house.
I totally accept that my dad is a grumpy old git, I’ve known him all my life and he always has been tbh, although obviously the frustration of being a once fit and healthy man who is essentially housebound and lost a lot of dignity has obviously made it worse.
They’re proposing to have a conversation about it over the weekend and I’m hoping that if my (much calmer!) mum is there it can be resolved. However, I’ve been wondering whether I should drop the neighbours a line. I’ve got no reason to assume they’re unreasonable people and I can understand why they’re a bit peeved at dad shouting to ask them to move. But what I don’t think they yet understand is that by making a tiny change and parking maybe 25m away, they would make a huge difference to my dad’s pretty miserable life. Dad has been asking me to pull up Land Registry boundary documents off the net for the conversation and I’m not optimistic that it is going to go well. I think he’s going to try and make it into a matter of legality when in fact it’s far more simple, it’s about an act of kindness they could do for him so as to make his (and my poor long suffering mum’s) life easier. I worry that if the meeting goes badly at the weekend they’ll be disinclined to be that kind to dad and as a result it’ll escalate.
The flip argument here is that I’m meddling, shouldn’t get involved and I’m just doing it because I feel guilty I can’t do more to help them out being so far away. I think these neighbours are decent people and I’d like them to understand but there is of course a very real risk that in sending a letter I could easily make it worse.
Mum is talking about electric blinds and things but they really can’t afford to be spending money on that sort of thing, and besides that would mean dad having to sit alone in a chair but in the dark as well . I know they shouldn’t have to park elsewhere just to please my dad (I’ve been on mumsnet long enough to agree that unless you own the land you have to accept people may park there!!) but at the same time the idea of having to spend a couple of hours 3 times a week with the sun shining directly into your eyes and no way of stopping it is pretty crap.
Any thoughts would be welcome, I’m really unsure whether to say anything or stay well out of it.