Just get a multi meter or test light. You could go down to the compressor too, just find the wire which gets +12 when the compressor is running.

One thing to double check... The wires which were going to the pressure switch go to pinns 87 and 30 on the relay, 86 is hooked to ground and the "Mystery wire" goes to 85?

That setup looks like it was probably from a fog light harness or something. The fuse can be eliminated (or changed for the lowest amp one you can find).

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.