anywhere, really. I added after Spriteset_Base.prototype.removeVideo so line 320.
This does not fix videos being deactivated; you 'd have to write some additional code to track which ones were playing and restart them upon return to the Scene_Map.
I had to do this for a game jam so I just used GalV's MZ Load Common Event plug it to have a common event play my video upon return to the map screen. It continues from the same frame it left off on, as far as I can tell.