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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.
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.
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!
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.
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.
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.
Just a bit south from the last part is a covered bridge that runs across the river. Check the shore underneath for another part.
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.
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.
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.