Ok.. the key On test was for the fuse test, not the starter jumper test.
If you have jumpered 12 v to the center terminal of the
plug on the firewall , then the problem is downstream from that test point..the reason we use that test location is b/c that eliminates all parts in the cabin as they are the 12 v feed to that point , which then feeds the starter soiinoid. However , you now want to look at the wires behind the oil filter for bad insulation and burnt wires ..These are the wires that connected that firewall connector test point to the starter..they were a common problem on that chassis due to bad insulation. Look right behind the filter area..check that before condemning the starter/solinoid.
And if these wires are OK, check the top battery cable nut on top of the starter for looseness/poor connection..that is another common 104 starter possible.
You may have bad starter brushes/sol , but check the feeds as mentioned before going for a starter.
You are zeroing in on it now...