I saw this unit while looking in a motorcycling magazine. It mixes two audio sources (ie both coming out the speakers at once) and provides automatic volume control based on ambient noise. I haven't spoken with the technical guy (he was out) but it looks like a promising product for those of us with GPS & voice turn instructions.

http://www.autocomamerica.com/store/details.asp?intProdId=22756&intCatID=890&intPageNumber=2&fldStoreGUID={0A3DA82D-E6B4-4719-A393-497CFEF75ED4}&fldCartGUID=&strSearchString=&intSearchField=

I'll post details on what I find out tomorrow (when the tech calls me back). Anybody used one of these?

-Zeke