I have had the same problem in the Alps. Mapsource does not want to use the small roads. I had to add a lot of intermediate waypoints to get the route I wanted. Once you are on Stelvio there is only one way to go anyway so another way out would be just plot a route to the bottom and then another route picking up from the other side.
Incidentally, I also found that tight Alpine roads can confuse the satnav. With many hairpin bends the satnav often thought I had U turned and was going back the way I came so a trip up Stelvio was accompanied by continuous "recalculating" and "do a U turn" messages. Eventually I turned it off so struggling to plot the route was a pointless excercise.