Interesting problem, didn’t even know that a “ Valet Mode “ existed but sounds like we should try and find out about it. I’ve found the following suggestions on the ‘net but I’ve no idea if they work, fortunately they are all very simple to try.
Using the remote fob seems a very simple option to cancel the Valet Mode, just unlocking with it does the trick. Other than that, you have to get the boot open so the boot lights come on. The boot lights are the signal for the system to cancel Valet Mode, the key in the door or just turning the boot lock won't do it. Also, if the switch in the boot latch has failed so the lights won't come on, the Valet mode won't cancel either.
Unlocking the driver's door with the black headed key should/might ? reset the Valet Mode. Easy enough to try anyway and at the least it may get you into the Boot so you can change the CD and release you from the purgatory of "The Best Air Guitar Classics in the History of the World (II)" !
Another idea I’ve found is that using the key to open the truck may also override/reset the Valet Mode. The key goes in a hole that is between the X and the K on the emblem on the rear of the car.
As I said I have no direct advice to offer so I’m just trying to help with things I’ve found as I’d be very interested to know the solution to this. There is quite a lot more stuff out there on this subject, if you find the cure please let me know.