well he has an after market radio so you do have to run a key ACC on wire ... other wise if it was jumpered it will be "on all the time"Of course it does or it would lose its memory. Duh. Some other modules draw a little, too, to retain critical settings, which is why a very small amount of power is always being used. That's normal and should be less than 50ma. In the PO's case, there is too much being drawn.
a hissing noise is normal that means the amp turned on from opening the door ( that happens on other GM cars and trucks as well)