New car stereo wiring question

The AM radio is not "hot." I'm sure he's talking about the memory "keep alive" wire. Get into the fuse panel. There are two busses, one constant hot, the other switched by key "accessory." If it does not have a dedicated fuse I would either pick a point in the box that is fused or add an in-line. There are several "hot" feeds, the courtesy lamp circuit, which might be easy to tap such as the glove box lamp, the brake lamp switch circuit