Starter Relay

If I was troubleshooting that picture, I'd do it like this:

I'd check the ohms from the soldered blank tab of the relay to the negative battery terminal. Low ohms means you have a good ground.

Switch your meter to DC volts and check and make sure there is a constant 12v on the big starter relay terminal that has the two wires on it.

I'd have a friend turn the key to start and I'd place my volt meter lead on the exposed wire that is pulling out of that connector. Should see 12v there because that is the start wire from your ignition switch.

Then with the key still turned to start, I'd place my lead on the tab of the starter relay that the bad connector is on and I'd make sure my lead was only touching the tab itself. You'll probably see 0v there because the power isn't making it from the wire to the tab, due to the bad connection.

Lets say there is 12v there but the engine still doesn't turn over. Now place your test lead on the hex screw with the key still turned to start. You should see 12v there because that wire runs down to the starter. If there is 0v there, then your starter relay is bad. That hex head should have 0v on it at all times unless the key is turned to start. Then and only then will it have 12v.