The furnaces I'm familiar with have four wires. Two are always-on 12V power (not switched short of the global battery disconnection switch) and the other two are blue thermostat signal leads (short them together to simulate a call for heat).
If you are testing the two non-blue leads and are showing 0V, you have a fuse or wire issue. It concerns me that you can't find a fuse marked "furnace." Sometimes one fuse is shared between multiple devices (my furnace fuse also feeds the fridge logic) but the standard Keystone label on the fuse is still "furnace."
If it were me debugging this. I'd reverify the presence of 0V, then throw a toner onto the lead that should be hot and a good frame ground, and see if tone appears anywhere in the fusebox. If it appeared everywhere in the box, then I'd take all the fuses out (in case they are crossfeeding the signal) and do the test again. That should at least help you to identify the fuse that feeds the furnace.
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2019 Cougar 26RBSWE
2019 Ford F-250
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