This confused me for ages but you need to head to Outpost Thorazan then head SE to the cliffs and head south towards the Imperial Base Outpost Zaroshe. BEFORE you get to the imperial base there is a Sand People area and the exact location you are looking for is called 'Banthan Drifts' The Coordinates to the cave entrance are -708, -893. Hope this helps as previous coordinates were incorrect and will send you way down the bottom of the map like I was for 20 mins