Been living with the issue for about 3 years now until last week, it was same ole same ole for the approach to get the Explorer to crank.

Working Theory: When the fuse box under the hood in front of the driver gets warm, the truck won't crank. The heat can be from driving it or even just sitting in the sun.
In order to get the truck to start, you can spray canned air onto the starter relay / starter diode area of the fuse box and the truck cranks 99% of the time on just one 2 second burst.

Last week, the truck just stopped responding to this approach of spraying canned air and I found that the starter relay was completely dead. After replacing the relay, we are back to the spray and run situation.

If the truck is cold and its cold outside, it starts every time without being sprayed with canned air. So during the winter months, the truck is extremely reliable.

Checked power to the starter and all is normal. Starter purchased 06/2018
Battery purchased 06/2018 and a load test Saturday showed it was in great shape.
Situation works or doesn't work with both keys so I don't believe this is a security issue. Plus the winter reliability also rules this out.

You can hear the starter relay click when the key is moved to engage the starter but the starter is not engaged.

Any thoughts on troubleshooting this issue? Does anyone possibly have wiring diagrams that would show me what to check from the starter relay onward or the systems at play in getting the truck to turn over?

All help is most appreciated.