I'm fairly sure I've seen someone post on here a link to a good website where you can download service manuals, wiring diagrams etc for domestic appliances.
I've got an AEG santo fridge ( big tall one, part of a pair with matching tall upright freezer ) ( not very old, but out-of-warranty ) and the self-defrost has stopped working. It's frosting up on the rear panels in a few days. The door seals look to be in excellent condition, and as far as I can tell, the door and cabinet are not distorted. Also, after opening the fridge and closing it, immediate re-opening is slightly harder, because of a slight pressure difference across the door which takes a moment or two to equalise. The manual says this will happen, and it does, so I think that is enough to confirm the door seals are OK.
So I need to look at the defrost timer / stat / element.
I've been able to get an exploded view of the fridge off the Electrolux / AEG website, but I can't see the self-defrost parts on it. I was hoping for a wiring diagram, before I go to the effort of horsing the heavy great lump of a thing out of it's space and getting in about the gubbins at the rear and having to reverse-engineer the thing by inspection and wire-tracing.