Trucks with BLIS have LED's from the factory. There is no bulb you can just pull out and put a new one in. What year/make/model is your truck? Can you show us a photo of your tails?
BLIS lights, when removed from the truck, have a board on the back. There's nothing to replace.
I have a 2019 Supercrew Raptor with 802A and Tech Package.
And I changed out the bulb myself.
I've hear the warning go off when backing out of my driveway and a passing car set it off.
Here is a just now taken picture of my driver's side taillight.
I just don't know what else to say except it was the upper bulb so maybe it wasn't the reverse light but the tail light hell you got me wondering now.
I guess that little strip in the picture is the led reverse light?
All I know is that after I installed the LED from Diode Dynamics it was brighter when reversing.