The frayed wire in the close-up (photo #2) is the general illumination circuit braid and is fine. There was no connection originally, no jumper is needed, you can remove the wire you put in. If you ever suspect a jones connector issue, try temporarily jumpering a pin with alligator clip leads and see if trouble resolves. If so, burnish the points of contact with small wire brush.

You say the light is dim? ALL lamps? or only 1UP lamp? is the lighting dim on all 4 "player up" lamps? IF SO, this is ball count unit alignment, the big rotary unit in the backbox with many spring-loaded contact pins. A small misalignment of discs results in bad connections like that. I had issues with mine recently, too.

The LOCK relays on all these pins always roasts to a crisp over the years and the lugs can fall off, but the connection is still good, most importantly are the contact blades it connects as this is general lighting power and the points may need cleaning if indeed all your lamps are dim. You can replace the coil for appearance sake, but it is not necessary if everything is working.

Great pin