Zereth-shahar is said to be situated in or on the "mount of the valley" (behar ha`emeq (Jos 13:19)). Cheyne (EB, under the word) says "i.e. on one of the mountains East of the Jordan valley (compare Josephus 13 27), and not impossibly on that described at length in BJ, VII, vi, 1-3." To the Northwest of this mountain is Wady ec-Cara, wherein there may be a reminiscence of Zereth-shahar. There is no certainty.
The International Standard Bible Encyclopedia
Mount of the Valley
The International Standard Bible Encyclopedia, James Orr (1915). Public domain, from the CrossWire SWORD module.