We don't have experience with OEM parts but I can give you some troubleshooting for your lights not working, though the Volvo dealer really should be able to solve this. I say that because you mentioned the trailer lights work on your other vehicles but not with your Volvo so we know it's on the vehicle side. That points to a harness issue or something else like a fuse.
But the first thing you want to do is take a look at your grounds again - I know the dealer said they checked it but... view full answer...
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