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Part #11 - Paleto Forest
Follow the highway just a bit past the town proper, and keep your eyes peeled to the east for several path bridges. You'll find this underneath one just off the road.
Part #12 - Paleto Bay
On the western shore of Paleto Bay lies a series of small islands. Walk out onto the first one (it's technically a peninsula, but whatever) to find a space ship part under a tree.
Part #13 - Raton Canyon
This is probably the hardest part to reach. It's along the buttress underneath the Great Ocean Highway Bridge. To get to it, you'll have to ease a helicopter into the small space between the buttress and bridge above. Good luck!
Part #14 - Cassidy Creek
Head to the train bridge that runs across Cassidy Creek. In the river below are a few rocky shores; this lies on one of them.
Part #15 - Mount Josiah
This isn't hard to access, but it is very tricky to actually spot. Take a helicopter or make your way up to the layered summit of Mount Josiah. Start descending the cliffs in a southwesterly direction, peering over each one as you drop. Eventually, you'll spot this part snuggled up on one of the edges.
Part #16 - Zancudo River
Head to where the Zancudo River runs between Mount Josiah and the Grand Senora Desert. Along the shore near the train bridge is another space ship part.
Part #17 - Zancudo River
Just a bit south from the last part is a covered bridge that runs across the river. Check the shore underneath for another part.
Part #18 - Tongva Valley
In the northern region of Tongva Valley is a large waterfall. Swim into the shallow pool at its base to find another space ship part.
Part #19 - Tongva Hills
This one's found at the Marlowe Vineyard. One group of vines grows on a separate hill to the south. Head up the hill to the tenth row of vines, and you'll find this along the path.
Part #20 - Tongva Hills
Two rivers run through the Tongva Hills; go to the southernmost one. Head west, and about one-third of the way down the river you'll find this cave (yes, it's very hard to spot). Inside lies another ship part.