I presume you are writing about the third brake light above the tail gate. I have a 2001, not sure if the same design but is LED. Remove the unit, should be a sealed one piece unit. A couple of screws and has a pig tail plug harness.If you have a voltage meter, test the dsconnected pig tail when someone stepping on the brake. If you have voltage, than you know the LED is burnt, very common, otherwise a problem somewhere else. I found mine used on Ebay for about $75.00, other wise over a $125.00 at the dealer.