well....when you look at the tail light, there are two bulbs that come on when you turn on the head lights. When I put my foot on the brake, the bottom of the two bulbs gets brighter. The top bulb does not change, however, that is the one that has the dual filaments..? (hope that makes sense)....

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2001 4.4 Titanium Silver, OEM Running Boards, Tinted Driver/ Passenger windows, Clear Side Markers, Clear/ Red Tail Lights, Axxis brake pads, and just added...Bosch Icons!!!!