The plugs are there for the factory shaker 1000 subs & amps (or they are in mine). If you pull back the side panel on the passenger side you should see the connectors for them.
1966 Mustang Sprint 200--Wimbledon White
1998 Mustang V6 Coupe--Silver--RIP, Totalled by brother
2005 Mustang V6 Premium Coupe--Sonic Blue