I really struggle with this myself. I found him. but its not quite easy to get to. I am using json. Therefore, if you use xml, you need to add "& alt = json" and then the end of your URL.
I found under an array of records. in the media ad $ group-> media $ content-> media $ thumbnail they have a list of thumbnails for the video. Each of them contains a video identifier in its path. Not the best solution, but it should work fine.
"media$group": { "media$content": [ { "url": "http://www.youtube.com/p/PLE7C605988F1AFDAA", "type": "application/x-shockwave-flash", "yt$format": 5 } ], "media$thumbnail": [ { "url": "https://i.ytimg.com/vi/[VIDEO_ID]/default.jpg", "height": 90, "width": 120, "yt$name": "default" }, { "url": "https://i.ytimg.com/vi/[VIDEO_ID]/mqdefault.jpg", "height": 180, "width": 320, "yt$name": "mqdefault" }, { "url": "https://i.ytimg.com/vi/[VIDEO_ID]/hqdefault.jpg", "height": 360, "width": 480, "yt$name": "hqdefault" } ], "media$title": { "$t": "[VIDEO_NAME]", "type": "plain" } },
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