The SP112 intersection is at a latitude/longitude of
N42 45.068 E11 27.472
  It is 5.3 km South of Santa Caterina on the odometer. This is a view of it approaching from the Santa Caterina side. (SS323 is the road on the right edge of the picture)

This is the view if you come from Scansano.  Sorry, but I don't know the exact distance from Scansano.

Turn East on SP112 and in less than 400 meters you will come to a fork in the road.  Take the RIGHT at the fork.  Coordinates for the fork are 
N42 45.021 E011 27.779

Procede down this narrow road for 2 km until you come to a gravel road on the left which goes up an incline.  Coordinates for this road are 
N42 44.386 E011 28.330
You may notice a tin can lid nailed to a tree on the right edge of the gravel road.

This road can be rough, so walk or carefully drive up the gravel road for about 350 meters until you come to a path on the right side marked by two stones in the ground an a tin can lid nailed to a tree.  You might even see pink marking tape on the tree.   Coordinates:
N42 44.256 E011 28.482

Enter the woods at this point and follow the trail for about 50 meters. You may see marking tape on branches along the way.

Soon you will come to the crash site.  It is a wide spot in a creek bed (usually dried up) with a pine tree next to the bank.  Under the pine tree is the monument.   Coordinates for the crash site are
N42 44.237 E011 28.467