Where is the fuse for the trunk light on the 2008 S80?

In a car that has been around for a while there is always the possibility of corrosion, or vibration created failures in the wiring or insulation. A test light or volt ohm meter would be helpful to determine that the voltage is reaching the led or light bulbs. If not then jumper the voltage from battery to light. It should illuminate. If it does not then the ground side is open. You will need to trace the wiring to the car frame. If the jumper voltage lights the lamp then trace the wiring back from the bulb to the fuse block to the firewall plug to the on off switch with the test light or volt meter. All along the way look for loose or corroded connections. Hope that is helpful.

fuse blown
fuse that look good can be bad, if they look bad, they are.
use any DMM meter, use Rx1 ohm range. check for fuse continuity.
if zero ohms , iTs good, and the meter never lies.
learn to use the meter.
if the fuse is good, use the same meter, to measure voltage
and trace the wires to the failing point. (open circuit)

check for bad fuse if you have multi meter you can check the switch ,place probes on each end of switch put the meter on low ohm setting if switch is good it should read 0.1 ohms when you activate the switch

TURN OFF IGNITION!!!!!
Look under hood and take off the cover on driver's side electrical box aft of air cleaner. Find the ECM fuse and remove it for a couple of hours.
Alternate is to undo battery ground cable in trunk for a few hours.it is off until the computer sees the problem again. so it there is something that needs to be fixed then the light will come back on

Hi em1cl, I suggest you check fuse #D3, 15A in the fuse block located in the trunk.

one wire comes from the dashes fuse block and the other runs to the rear lights. an ohm meter can help you find it

Check for Poor Connector in Trunk the GROUND WIRES GET Corroded and also the connector for the rear lights should be in trunk on the Drivers side

check the brake light fuse also check the brake light switch adjustment and continuity with a ohm meter to make sure it is opening and closing with brake pedal applied

Once you remove the Right hand side tail light assembly (by removing 3 wing nuts from the trunk), look straight down (from the tail light opening). You will see the top of the muffler and there is a wire harness right top of it. Check for wear/heat damage.

it is a process...testing powers and grounds to find short/open .you need test light , ohm meter , and good wiring diagram to continue