Just DuoLingo, watching Japanese stuff without subtitles (including cultural guides for further understanding) to force myself to listen, and then looking up words I don't know when I come across them.

My goal was never to learn how to speak or write anything; just understand it well enough to watch Japanese media in the original language. I still often have to rewind and play back stuff to get it all. Especially if they are saying a long sentence really fast.

It also probably helps that Language itself is one of the things that tickles my autistic brain.