The rubber seal that runs along the entire length of the firewall. Seals the hood when its shut. You can remove it, but Bnoon recommended that you leave a little strip in front of the cabin air intake to prevent engine fumes from being sucked in. Allows heat to escape out the back of the engine bay. Seemed to make sense to me...been 10,000 miles and no adverse effects. Works on any car...4/6.
