That would probably be where the amps get their power for the sub enclosure that mounts there on cars equipped with the Shaker 1000 stereo. The shaker 1000 cars had a sub box in the trunk behind the right taillamp. There is a pair of amps that bolt there.
Picture borrowed from crutchfield.com
1966 Mustang Sprint 200--Wimbledon White
1998 Mustang V6 Coupe--Silver--RIP, Totalled by brother
2005 Mustang V6 Premium Coupe--Sonic Blue