Same thing happened to me on the way to summer splash I didn't have a bed floor at the time and it was a rock from the back tires.
yah i had my s10 back window replaced in my driveway for like $175
Hey I've got a solid back glass for an s-10 and I could bring it with me to dragfest.
Let me know if you are interested.
He has open fenders and no bed cover it was prolly a rock from you tire as stated above, new back glass in and leak free 35 bucks (do-it urselfers special!) off to dragfest we go!