Power being grounded
I feel like Scott, I think that a power lead to the radio has accidently been grounded. I'd relook over the installation of your new tune box as well as your schematic of the car and tape off the appropriate wire.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan