We were in Argentina for a month a few years back and traveled through many areas where the rally was going, It's very hard to know exactly where you will get to see some action.
They can and do change parts of the route often, the overnight stop points are a sure way to see some of the competitors vehicles, but the action stuff is harder to get info on.
We were lucky we stumbled on a fuel stop on one of the transport sections so we saw a lot of the trucks and many of the support veichels.
If you know they are going through a pass between to overnight stops it's possible to find some acion.
We met a group of German bike tourists that were very frustrated how little info they could get on the race sections. They were going to camp near the border crossing at Passo San Fransisco because they new the race was going through the next day.(pic below)
They may have made it easier in recent years what information are they giving now in Peru?