Tach adapter

I tried to instal an RPM tach a while back , the installers told me they cant do it on a 2004 GT , so when i was surfing the net this afternoon i came accros this....http://partshopper.com/fordmustang/itemdesc.asp?CartId={01BA167C-827D-40C9-BE6D-1AEVEREST6C4DEBAD2D}&ic=PEP%2DTACH&eq=&Tp=
Its a tach adaptor, will this make the instal of my RPM tach possible??
anyone know about this.and how does it work ?

thank you

damn no one knows about this!!!!!....