New truck, new problems

There is a ground wire that is supposed to run from the back of the engine to the (edit) firewall. If that is missing the speedo cable becomes the new ground path until it burns up.

200k miles indicates bad brushes in the alternator. You can tap it with a wooden hammer when it isn't working and see if it starts to.

I replaced the wire from the alternator to the big fuse block and from there to the battery and it seemed to have fixed the problem.

The Spero cable is jacked and I found where it is jacked. About 6 inches from the tranny. I got a lead on a new one.

Good work, Glad you're getting her straightened out!!