Remove the brake light. Flip it upside down. Buy some silicone caulk (the clear stuff). Put the caulk all over the underside of the light and anywhere pieces seem to be touching. Be sure to not get caulk into the hole where the power cable plugs in. Let it dry for plenty of time. This method worked for me. If you still have a rattle, put some more on there.
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Nick,
'99 Contour LX, Zetec, ATX, Light Blue:
Aesthetic: sidemarkers, badgeless, custom mesh grille, interior neon lights, CEG stickers, Radar Detector wired to come on w/ ignition
Performance: KKM Intake, rez removed, 2 1/4 inch straight pipe muffler, Focussport wires, Accel super EDIS coil, more to come
Audio: No back seat, Lots of Dynamat, Panasonic CQ-DF800U, 1700 watts of Kicker Equipment:2 ZR 360's, 1 ZX460, 1 ZR240, 2 Solobaric L7 12's, 1 resolution component set, 1 pair of impulse 5x7's; All wiring by Phoenix Gold; Security: By DEI Python 1000 ESP