[HardcoreZ28]

I'm putting good speakers into my T-bird and plan on keeping the stock radio functional but I'm also going to hide a modern headunit somewhere under the dash or seat. Both will be playing out of the same speakers. How do I wire that up so I don't get any kind of funny distortion between the two? Obviously I won't have both head units running at the same time. I think I need some kind of relay switch off of the remote wire in the modern unit but I'm not sure.