The locations are not specifically listed in the official Guidebooks for Guests with Disabilities. I'm not sure why, other than that allows them to be changed without the Guidebooks being out of date. In actual fact, they haven't changed them in a long time. The Guidebooks do caution that " These areas are provided for Guest comforts and are filled on a first-come, first-served basis. Guests should plan to arrive early, as space is limited. Be advised that most areas along our parade routes are accessible to guests with disabilities. Due to Guest demand, viewing spaces cannot be reserved or guaranteed. Parties larger than six will be asked to separate and reunite afterwards."
If it is very busy and the space is getting full, you could be asked to split into a party of even smaller than 6. Once you have gone into the space, the person with a disability needs to stay there in order to save your spot.
The viewing areas are marked on the park maps with a wheelchair icon and the letter 'P' and that is your best source of locations. (Actually, seeing the locations listed below, I'm thinking that may be why they don't list the locations in the guidebook - without a map, knowing the locations don't help a lot).
At MK the areas are as Michigan mentioned and on Main Street in front of the Train Station on the side closest to City Hall. It's the same sites for day and night parades.
At the Studio they are in front of the giant Sorceror's Hat (on the left side as you face the hat) and in front of Keystone Clothiers on the main street (Hollywood Blvd.)
At AK the parade spots are at Discovery Island across from Beastly Bazaar and another in front of Pizzafari. There is also on in Asia at a sort of dock area across from Flights of Wonder and at Africa in front of the Dawa Bar (which is by Tusker House restaurant).
At Epcot, there is no parade, but there are some viewing spots for Illuminations. The first 2 are just as you are coming into World Showcase; there are 2 shops as you come into the area. The two viewing spots are next to those - one on the Canada side and one on the Mexico side. The other spots are in front of Canada (sort of between Canada and the UK) and in front of Germany.