12 volt source

I'm putting on the relay harness for the HID's and want to install this harness in a way that it doesn't look crappy.I noticed that the 12v "hot" goes into the engine fuse box for most fuses-it has a nut on the supply wire which I can hook up the plus side of the relay harness but I can't figure how to get the access cover off the lead in wires. There is a snap tab on the front,but there is also something else holding this cover on.Where is the second attachment? I know some have gotten access to fuse box for their super amps-sound systems. Any thoughts?