From my understanding....(repeating what frisch already said)
Shaker 1000 has 2 subs in back(you could easily see them in the trunk) and two in doors...
Shaker 500 has 2 subs in doors
Regular radio has no subs
If I am wrong someone correct me.
Anyway, once you figure out what it was, go to Crutchfield.com, make an account and then put in your vehicles info, then add a radio to your cart, a screen will come up saying you need _________ to add an aftermarket radio in your vehicle, that is what you need.
I have an 06 v6 coupe with shaker 500...this is what they said I needed PAC Ford Radio Replacement Interface Allows aftermarket radio installation in select 2005-up Ford-built vehicles (model C2R-FRD1) at Crutchfield.com
But I am not sure if that is the same part you would need for the shaker 1000 or stock radio...I am almost positive it is but I don't want to say yes.