Boss spawn point is just north above the arrow (top tip on the right). Note that the current map is not entirely correct. See also.
This boss does not begin his patrol until you find him. He can be found down the road that leads east in the center of the island. He also shares his spawn point with Maxine Coldstone. If you see Maxine, Harn will not be on the map. Zone and try again.
'Finding' Harn does not necessarily mean that you have been able to see him, so it's possible that he has already left when you arrive at his starting point.
You can use the knowledge about the path he patrols to your advantage. Invoke his patroling route by coming into his radar range, then turn back towards Collector Gabrian. Further up the slope the path will fork; choose the right path. You'll need to fight small groups of Dark Flame Dryders and maybe one or two Drakes along your way. Once again into radar Range, Harn will "patrol" right to the position you are waiting for him after all.
It rarely happens that he gets stuck along his patrol, in which case you will have to look for him.
You can also circle around and approach the spawn point from the north, making him run into you immediatly upon spawning. This would take longer then the above but if you're going for other bosses as well or vanquishing you can save the trouble of cutting him off later on.