I believe it is for the antenna not sure. It can be moved though. I will be getting 6.5" speakers installed this weekend with my system in the rear deck. Since they have to cut even for 5.25" then I might as well put 6.5" back there.
ah. perhaps the amplifier. good thought. i saw something in the manual about an antenna amplifier. (which them made me wonder if this was why people's reception was so bad; maybe it's defective or not "plugged in").
Found out the other day the box is the timing device for the rear window defogger. I was told by my Subbie tech who went to class and asked especially for me since I sent him with a list of questions that we wanted answers for. SFRinNJ