I just prepared one for another customer and I have 3 more that I can extract out of existing good cores, which accounts for a $25 cost. This wire doesn't go to the fuse box, but to a two wire red colored molded female bullet near the headlight connector and tail-light connectors. 1970 is the only year that has this female bullet in the underdash harness for direct connection. This wire is embedded in the tail-light harness braiding, so all of the braiding has to be cut away for me to extract this single wire. Once out, the only thing you can do is tape it to your existing harness or send it to someone who can re-braid the entire harness. Rhode Island Wiring is one company that I know of that can do that.
If you want one, simply PM or e-mail me.