I have grounded the Head Unit to a point in the trunk and the sled to the ground terminal for the original head unit. Thus, I have two separate grounds for the rio: one for the HU itself and one for the sled.

What is interesting, though, is that when i take the RCA's leading from the Rio to the Amps and plug them directly into the Sony, I get no whine at all. (Bypassing the Rio Aux)

So perhaps, as I am now thinking about it, the ground wire from the Rio HU and not the sled, is compromised and I shoud re-run that?