Audio Systems: radio CD player install to 91 Nissan p/u, nissan d21, fuse panel

Question
I have a 91 Nissan D21 pickup that had no audio installed at the factory. I want to put in a basic radio/CD player. I have the connectors that work - there is a plug under the dash. I have tested with multimeter and there is power on the pin for ACC (ignition switched) but there is no constant 12V from the battery on any other pin. Where do I need to look for the source? I see an empty fuse spot in the fuse box under the hood, next to the horn relay, but it isn't marked and I can't tell if there is a wire running off it. I haven't been able to find a diagram of this vehicle with or without a factory installed radio. Thank you,

Answer
You can use fuse tap (found at any auto parts store) to tap off a fuse in the fuse panel that has constant power. Be sure to use the hot side of the fuse. Remove the fuse and see which side of the socket is hot and tap from that side with the fuse in palce. Use an in-line fuse of about 10 amps unless it already has one.