Now that I think about it I don't know why I took it for granted you had a Mk-III navigation computer. If that is the case then all said above applies.
The Mk-II nav computer had a separate GPS unit, which later series III and IV integrate in their housing. Thus, if you have a Mk-II some additional messing is required with the GPS antenna lead. It is no big deal though, and there is a do-it-yourself tutorial out there for just this case.