Anywhere metal, and the magnet handles the rest. Popular spots on a vehicle are inside the rear bumper, a wheel well, along a frame rail or up in the undercarriage. Inside works just as well. Glove box, under a seat, or stuffed in the center console. GPS signals pass straight through plastic, glass and fiberglass, so a hidden gps tracker loses nothing sitting in the cabin. On a trailer, most owners go for the tongue box or the frame. Equipment tracking usually means the battery box or somewhere under the deck.
One mistake to avoid. Burying the tracking device under solid metal blocks the signal. Everywhere else, the magnet holds at highway speed and over rough roads, so the device stays put.
If you want a more detailed answer, we put together this article about the best spots to find hidden GPS tracking solutions:
https://spacehawkgps.com/blogs/post/where-to-hide-a-gps-tracker-in-a-car