simple question , if i have a GPS chartplotter , with an external antenna , can i get a T fitting in the antenna connection and use the same antenna for a GPS speedo mounted center of the dash (where passengers can see our speed) ?

It's even simpler. I think it's the brown wire. Look at the directions for wireing and the is a GPS out put wire. That hooks to your speedo. You then have to configure the GPS out put for the same language as the speedo. It's got to be set to NMV.... I forget the exact initials. But it's all in your owners booklet

thanks , as always this place is a wealth of knowledge