Before replacing your LEDs or pieces of your harness, consider these steps.
When to perform this test: if your LEDs light up with the parking lights, or with the brake lights, but will not sequence or only one or two bulbs sequence.
Test #1 (1996-2004 ONLY) - Install the sequential tail light harness on one or both sides, and in the harness install the factory halogen bulbs. Then move the turn signal or 4-way flashers to see if the brake lights with the harness are sequencing properly. If they do not sequence properly check to make sure you swapped out the modified turn signal flasher.
IF the flasher has been changed and it still does not sequence or does not have the flasher (2005 and Newer) then the harness is defective.
IF it does sequence properly with the halogen bulbs, but not with the LED bulbs proceed to the second test.
Test #2 - take the tabs on the LEDs and bend them out away from the LED. Also, take the tabs on the sockets and bend them in towards each other. This should be done gently and very slowly, because the tabs are very flexible and easy to break. The best tool for this is a knife or a set of small needle nose pliers, but be careful and make sure the keys are NOT in the ignition to prevent any shocking hazard. Because the LEDs draw much less current, it needs a much tighter and better contact to transfer the lower current to allow the bulbs sequence and flash properly. This should solve the sequencing problem.
IF this does not fix the problem, replace your bulbs.