Find the rebel leader is a quest in Back in Action.
For that, you're hinted to ask locals about his location.
Ira won't thust you at first and will task you with liberating the mines near Cambria. Once this has been done, she will tell you that Miguel is in a hideout north of the road in the mountain near Estoni.
There, you'll just have to talk to Miguel to complete this quest.
Note: you can directly go to the hideout to talk to Miguel and ignore father Walker and Ira.