I'm reading your reply and the first check is at the starter solenoid with the test lamp.
If the light lights on the large terminal...And then you have someone attempt to start by the ignition key with the test lamp on the small terminal at the solenoid, and the light lights there too, you can assume that the system is operational from there.
The test lamp should illuminate when the key is cycled to the start position and Stay Lighted. Does it? And is the light on the small terminal of the solenoid as bright as it is on the large terminal? A dim light could mean it's not enough to engage the solenoid and starter.
This test tells us whether we are getting the proper voltage signal to the starter in order to engage the starter.
Another Test that can help determine if there is a unwanted voltage drop is to place the test lamp from ground to the large terminal while someone is attempting to start the engine and make sure that light stays bright during attempt to start. The lamp should stay bright or the starter should engage and begin to crank the engine.
If you are getting a good strong 12volts to both terminals of the solenoid...Full time on the large and solid 12volts on the small wihile key is moved to start, then there is one of two problems.
The starter does not have a good ground or the starter/solenoid is bad.
If you have had the starter tested, and they tell you it is OK, my next question would be, did they test the solenoid and starter as a unit?
If so, then you need to be sure the Engine has a good ground cable between the engine and frame or negative terminal of the battery.
Back to Basics:
Make sure the battery cables and thier terminals are clean and making a good connection. Make sure the Cables are in good condition. Make sure the Ground connection to the Engine Block is good and clean and tight.
By the Schmetic, you would not get a 12volt signal to the small solenoid terminal if the Fuse Link was bad. Additionally, if the fuse link were bad, the starter relay would not click.
I hope you are able to isolate the problem from here.