72 Demon fuse block wire

I'll tell you what might be. That black (how large what gauge?) might be a hacked feed to the "hot" buss in the fuse panel. If so, it originally came from the welded splice as I mentioned. Look on the rear of the fuse panel. There are 3 distinct separate busses. One section is "accessory," powered only when the key is in ACC or "run." Another is the "hot" buss and is always powered. The last, at one end, is just one fuse and is for the cluster/ dash dimmer controlled lighting. THAT fuse gets its power ONLY when the light switch is either in "park" or full on, from the tan wire coming off the light switch.