I have 2 racitve mufflers on my SVT. Check to see if the mufflers are hitting any where near the rear bumper. I had to cut out some of the plastic on mine. That could be it.

98 SVT
kkm intake
KVR Brakes
shaved trunk lock and ant.
removed: ding gards, badges, dealer sticker, mid muffler
Cusrom Stero Install: Dyno matted trunk, Sony deck, ADS amp and front, Orion subs, Sony amp, Optima Yellow top
Custom grills