It doesn't exist.

I found the body and the key a few times, I believe it's a random event. I never actualy got around to investigating it though, but I do recall some directions in there (something like the southwest corner, or something) check the jumkyard, or the corner of the map, or anywhere a lot of cars are.

Yeah in junkyard. Its in a burned out bus but it not really worth going to because all you get is a couple of skill books

Yes, finding the body is a random event. I've only gotten it once myself. If I remember correctly the bus is in the northwest corner of the junkyard you find Dogmeat in.

The key is for a secret supply box found in a bus hidden in the Scrapyard. .If you do not have the key , it is very hard to lockpick the box so I recommend finding the key.The box is , I believe, in the north-western corner of the Scrapyard. The Scrapyard is south of Minefield.
Hope this helps.

Its a random event you will get it is in the scrap yard i think its in a south part of it the area with raiders in it