The car that was driving in front of me stepped on a small rock, which then came flying to my car's window and cracked it.
こんにちは。
ご質問ありがとうございます。
"The car that was driving in front of me"=「私の前で運転していた車が」
"stepped on a small rock,"=「小石を踏み」
"which then came flying to my car's window"=「それが私の車の窓に飛んできて」
"and cracked it."=「ひびを入れました。」
ご参考に。