You didnt hear this from me, but....
If your P-800 is an 80's or early 90's vintage GMC model, it has a toggle switch above the bulkhead door for the cab and package compartment lights.
Its a "3-way" switch, with 3 blades on the back side. The middle blade is always hot. You can run the wires from your radio up above the door, held in place with a bulldog clamp, and connect your hot wire to that middle blade with a roach clip. The metal plate that the toggle switch is mounted on is grounded to the body of the vehicle, so your negative wire can clamp on to that with another roach clip.
This is a safer method because you are hooking into a circuit that has its own fuse, so if you have a brain fart and get the wires crossed you will blow a fuse instead of starting a fire. Do not ever hook anything directly up to the battery.
The better choice would be to not hook anything up at all, but if you are gonna do it anyway then make sure you do it right.