On my 2006 Discovery the fuse box for the trailer plug is in the right rear compartment where the spare tire should be located. They are in a black box with all the wires going in. Turn on your turn signal and listen for the clicking to help you find it.
2006 Discovery 39L-2008 Honda CRV
Bud, Sharon, Cody (Grandson) Sophie, Attack Trained Cockapoo