Where is the A/C relay located?

It is built into the intelligent power module and is not a replaceable part. To replace it you have to replace the complete IPDM.

The fused power through the relay is the only part of the circuit that is accessible from the outside, but there is no way to turn it on and off without the scan tool. So, you can test the fuse and jumper power to the compressor but there is no way to trigger the relay to see if it works without a scan tool that can access the HVAC system. What is the main issue? A/C not working? Have you done a pressure test to see if there is a charge in the system?

Yeah too much is as bad or worse. I would get a set of gauges and attach them and see what the pressures are. As a rule of thumb, a system that has a charge in it when not running should show right around the air temperature. So, if it's 98 degrees you should see between 96 and 100 psi static pressure. Then start the car and turn on the AC on max high and full blower. Now check the pressures and compare them to the chart below.
So, say you are testing, and it is 85 degrees out. The low side should be around 50 psi and the high side around 230 or so.
https://www.2carpros.com/articles/re-charge-an-air-conditioner-system
Now if you see the low side is say 70 and the high side is 300 or more, it is overcharged. Then I would take it to a shop, have them pull the charge out and put in the correct amount which will be on the tag or 1.21 pounds,19.4 oz. or .55 kilogram. Then see what you have.