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First off my alldata software includes up to 2009 any cars, the diagrams below I found them on google.
Rockauto shows that your 3rd brake light connector can have either 3 or 6 wires. Make sure yours has 3 wires before you proceed to below.
The 3rd brake lamp (BN=brown) and cargo/bed lamp (VT=violet) are on totally different circuit. If the BN is short to power, how in the world can the VT, a different circuit, is also short to power at the same time, the odd is next to zero at the 3rd brake lamp connector and highly likely at the BCM. Let say VT or BN wire is short to ground at BK/YE (black/yellow) at the 3rd brake lamp connector, next time you step on the brake or turn on your bed lamp, you will blow the fuse. Now say VT is short to BN, but both wires won't have any 12v feed when engine is off, so anything short at the 3rd brake lamp connector is just not possible when your 3rd brake lamp connector has 3 wires. Spend your time only at the BCM (never forget it's hot at all times), not at the 3rd brake lamp connector and not in the middle of the VT and BN wires.
The issue appears to be at the BCM, it's located under the glove box near the passenger kick panel, so check its connectors there first, terminal 13 at 2280E and 26 at 2280C respectively. Then you can do further testings using ohmmeter and voltmeter in different ON/OFF conditions to find out where the short to power is. If there doesn't seem to be a short to power at all, before you give up and buy a new or used BCM, and reprogram it, you might want to do a reset first. Check google on how to reset your BCM, if your scanner is not capable to do it.
Above diagrams include terminal 13 and 26 (what you need). Download 2280C and 2280E to view full pinout in PDF below if needed.
