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IDFWU 420-
Sorry I wasn't able to reply to your inquiry in time.
You really need to determine how to access your OEM HU's diagnostic screens, so you can verify that it's actually seeing the reverse gear signal.
autoharnesshouse offers an extra wire with terminal to add the reverse gear signal to cars that didn't have the factory wire (such as FR-S). Did you add that? The terminals they use are Chinese. They are not the proper Japanese terminal for the connector housings that Subaru and Toyota/Scion use. It will mostly work, but potentially won't seat fully or properly. That could cause intermittent problems. Perhaps when you switched to your friend's BRZ HU that terminal makes contact, but with your HU it doesn't? (I know, low likelihood that it would consistently fail with your HU and not his).
The other thing to check is whether your HU is successfully powering the relay. The HU will do that for several seconds at each boot up, regardless of whether the reverse gear signal is connected, and regardless of whether the HU sees video from the camera. It's possible that your Scion's HU isn't able to supply enough current for the relay they used, and/or that your HU's 6V camera power output has gone bad.
Yet another test is to temporarily power the camera from 12V ACC (so it's on all the time your key is on, bypassing the relay). See if that makes your HU happy.
All of that should help narrow down the problem.
BTW, what are the model numbers on your Scion's OEM HU (model printed on front, and Toyota and manufacturer's #s on label on top)?
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