'08f150stx

I ask this because I added a HD ready head unit and have the external hd tuner.
The install instructions say to keep it away from heat and moisture.
That is about it for instructions.
I was installing the head unit, looked in the area behind where the head unit would be, and there was not room there.
And need an unswitched 12 volt power source.
Any ideas where is a good place to attach it?
I figure I am going to attach the tuner with velcro.