If you pull the relay you can perform a test to determine if it is stuck closed, if you have a multimeter. It still would not explain why the horn isn’t sounding if the relay is stuck closed though. That would be a separate investigation.
From what I have seen with lots of car electronics they are ground/negative switched. That could explain why you measured 12v at one side of the coil. I suspect pressing the horn button on the steering wheel causes the other side of the relay coil to tie to battery negative/chassis/ground.
One thing that did not make sense in the relay resistance measurement is that the coil is only 80 milliohms. I would expect the coil resistance to be much greater since only 10’s of mA should be required to energize the coil. I would have expected the coil resistance to be in the 100’s of ohms.
I’m also surprised that the relay is normally closed. That means with the relay plugged in that 12v is being fed to the horn when the relay is not being energized. Something isn’t adding up.
Leave each fuse in place. Put the voltmeter in mVdc mode. Measure voltage across each fuse with the Pilot off and key out of the ignition. The fuses with the largest voltage draw across them are the ones I would pull first.
There is a rear AC fuse for the rear blower motor.
If your aftermarket GPS was connected to an always hot/on circuit that could explain why you had the problem of the battery draining.
If you need to reconnect it permanently you could hook it up to a switched circuit (one which is energized only when the key is turned in the ignition).
Glad you found the cause of the problem.
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