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One thing you might check is the Park Aid module. It is located way up high behind the passenger kick panel. If any moisture gets in thru the cowl (very common) it drips right on the module connector and runs inside along the board and ruins it. A new module will come with the short harness and connector to replace the corroded one. When I replaced mine I slid it inside a ziplock bag with the opening pointing down, you can still hear the speaker fine thru the ziplock bag.
From what I have read, the sensors can go bad or a connection disconnects or is corroded. Guess I would start there to eliminate those possibilities.
I was going to get some test values for you from my service manual but there are a lot of steps to go through to test the system. If you want, I could try to PM you the pages. Let me know.