the car has a bumper to bumper warranty that will be voided if I replace the head unit.

Are you sure? Does it actually say so in the warranty contract?

That sounds like something a dealership might tell you but isn't actually in the text. If it's actually in the text, what's the exact wording?

I could understand if it said that replacing the radio voids the warranty on the radio. Or that the warranty doesn't apply to aftermarket radio components. But voiding the whole car's warranty by replacing the radio? I don't think it's legal for them to even suggest that, let alone make it a policy.