So for my constant power, I used the relay box in post #3. That relay box has 2 relays with 3 fuses. 2 fuses are for each of the relays (1 for each relay), and then there's another fuse that is constant power.
That relay/fuse box is plugged straight to the battery (actually to the starter relay post). Coincidentally, the constant power wire for the relay/fuse box is yellow so I connected the yellow wire from the radio to the yellow wire to the relay/fuse box. Worked out perfectly!
If you don't get a relay box like I did, then you'll have to go straight to the battery. Although, there are points in the factory harness that has constant power. For instance, the cigarette lighter has constant power. Well, it does in my '74 Duster.
