Most likely: Rear E-brak cable is rubbing against your underside - there is a rubber boot that keeps it from squeaking.
Next in line: Your back seat. Supposedly the foam on the underside of the lower seat can rub against the metal frame.
Highly unlikely: Struts have lots of dirt worked in them and squeak.
E0 #4384 of 6535
Born Jan. 9 1998
My SVT from the ashes
147,466 miles, purchased Nov. 2002 @ 76,559 miles