Sounds like the Kenwood isn't well shielded against RF interference. Probably not much you can do.

Assuming that the antenna wiring is all properly shielded, the only other suggestion I'd make is to try reversing the order of the two units in the dash. Maybe that will reposition the RF-sensitive components in the Kenwood.

Oh, hey I just thought of another possibility. You said you've got an aftermarket antenna adapter for the VW powered antenna. Maybe that aftermarket adapter is what's not well shielded. Perhaps there's a way to shield, move, or reposition that.