Unfortunately they got rid of the disabled viewing area years ago. I do ask every trip in case they've reinstated it, but so far they haven't.
However. Best kept secret. The BEST place to watch the parade is coming up and round from Adventureland. Pass Splash Mountain, and there are big gates just before BTM. If you time it right they'll have started putting up the barriers, and the corner there is absolutely perfect if you're in a wheelchair or
scooter. DD and I had a perfect view of the parade as it came out from the back: I was able to turn the front of my scooter round so it curved round the corner of the rope and I was sitting far enough forward to see. Normally you're a long way back from the front wheel (which obviously mustn't protrude) and then you can't see anything.
We've also watched from opposite the buildings just past Pecos Bills, just before it does a right turn to go over the wooden bridge and down into the hub. We chose a spot where the rope was so close to the wall that OH and DD were able to sit on the wall and I parked in front of them with my scooter lengthwise and turned the seat round to face the front. Mind you, that particular view is very reliant on not having selfish pigs leaving right over in front of you with their cameras and not only spoiling your view, but also placing themselves centre of all and any photos/videos you try to take!
Finally, DD and I found another great spot just behind the castle next to the wall, where we watched the Halloween parade. Again, i got into the corner where no-one could get in front of me, and once the walkway across had been roped off, had a brilliant view of the whole thing. Would have been even better had a couple of CMs not been standing against the rope themselves blocking my view! I sometimes wonder if they do this on purpose ...