Starter Problems

Or you can test the cables to see if they need to be replaced.

You can get a voltmeter and put one probe at each end of the cable you are testing- crank a few seconds and there should not be more than .5 of a volt reading. You are reading the voltage within that cable which would be the voltage drop from one end to the other. You'll have to isolate each cable the get a true reading for that cable. Try that same process for the entire starting circuit to see if you have any resistance issues before throwing parts and money at it.

A faulty starting system will shorten the life of a starter. If you've replaced that many starters I'd look at the starter wiring circuit or you'll forever be dealing with starting issues.

Jumper wire likely melting because the wrong terminals are being shorted. You want to jump the little "s" terminal to the starter input (positive) terminal.