1. Buy a meter, which will be cheaper than replacing parts without knowing what's bad 2. Put the neg meter probe, starting first test under load on the battery negative [with + probe on the battery +], and then test at the starter ground to ensure that you have a good ground for the starter. 3. While trying to start [under load], go down the line with the + meter probe, keeping the - probe on the starter ground... starting with the battery post, then the battery connector, down the line to everything in the start current path, ending up at the starter. 4. The point you see the voltage drop substantially when under load will identify where the problem is
Electrical Troubleshooting 101