An interesting electrical puzzle

My car had a couple of electrical issues recently, which led me down the path of checking all the wiring at the fuse panel. While I was there, I discovered something puzzling I'm hoping one of you can shed light on. I have an April 1973 Euro car, 06236. The wiring at the fuse panel appears to be almost entirely original and in very good condition. In addition to the horn, lighting, and radiator fan relays, I have a mystery extra relay, visible at the far lower right of the attached photo. It's a Sipea brand, with standard DIN terminal markings for an SPST relay. It looks of an age with the other relays, as do the wiring, connectors, and connector covers. So I can't know for sure, but it seems factory, or at least "old", and not obviously hacked together by some subsequent owner. I had always thought it was an alternator relay, but it turns out it's an odd radiator fan relay.

The radiator fan relay at the top right is a standard Lucas 6RA SPST relay. W1 (DIN 85) looks like switched power, yellow wire, W2 (DIN 86) is connected to the heat switch at the radiator, green wire. C1 (DIN 30) is a red wire coming from the radiator fan fuse, and the C2s (DIN 87 and 87b) are each connected to a blue wire. One of those blue wires heads directly to a radiator fan, but the other goes to the 87b terminal of the mystery Sipea relay.

The mystery Sipea relay at the bottom right has terminals as follows: 85 connected to a cream colored wire that disappears into the main harness at the firewall. 86 is ground. 30 connects to the passenger side radiator fan. 87b connects to the radiator fan fuse, just like the main radiator fan relay C1 (DIN 30) terminal. 87 connects to a blue wire that ends at the Lucas relay C2 (DIN 87) terminal.

The behavior is such that when the Lucas relay is energized, both fans turn on. If only the Sipea relay is energized, only the passenger side fan turns on. But I don't know what's energizing the Sipea relay and why there's the odd blue wire connection between the main fan relay C2 (DIN 87) and Sipea relay 87 terminal.
I haven't done further digging - any ideas what problem this design is trying to solve, and why it bears no resemblance to my 1973 Euro owner's manual wiring diagram or any other wiring diagram I've seen?
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I've done some more debugging. Both the main Lucas relay and the mystery Sipea relay work correctly. Each radiator fan has its own power wire; they aren't in series. The two relays are ganged together but in such a way that the passenger side fan can run independently via the mystery relay. The passenger side fan gets power either from the main relay or from B+. The really odd thing is that the mystery relay's DIN 85 signal line doesn't seem to matter. I unplugged it and both fans still came on when the main relay was energized. That makes no sense - the whole point of a relay is that it doesn't switch power without a signal, right? In any case, It looks perfectly straightforward to remove the mystery relay and connect passenger side fan power to the main relay's second C2 terminal. But I'm reluctant to do it without knowing why that second relay's there. It looks awfully intentional: Italian relay, Italian connector covers, wiring from the harnesses. It would help to know what the other end of the DIN 85 wire is connected to, but it's one of many, many wires heading into the firewall and tracing it will be challenging.

 

I wondered about that, but the wiring doesn’t make sense yet. I’m still trying to figure out how the Lucas relay can activate the Sipea relay while the latter’s 85 terminal has no power, which appears to be the case. I must be missing something. It’s like the Lucas relay can switch the Sipea relay on but the Sipea relay can switch itself off. Very odd. And so far it seems like a unique setup. The car is an Italian market car, so I wonder whether early in its life an official service place went to some trouble to put this together. The wiring looks quite nice. If any of you have Euro cars after 06236 with this setup, raise your hands…

 

I don't have A/C, so it's not A/C related.

The front two red fuse box inputs have a connecting conductor underneath, so it doesn't matter which one the black wire connects to. The remaining wiring looks identical to mine (and different in various annoying ways from all the wiring diagrams I've seen).

This is very interesting! Two Italian market cars with the same relay setup, so likely factory-installed. But nothing on the wiring diagram, nor how the system really works.

 

Any idea how the system works? Otherwise I have a weekend of fun with a multimeter ahead of me. I hate unsolved puzzles.

 

After a bunch of time with a multi-meter and a circuit tracer I think I know how the fan relays work. Looks like my car is pre-wired for A/C even though it isn't installed, and the second radiator fan relay behaves as Steve Magnusson described above. I toned out the wire from the Sipea relay's DIN 85 terminal and it ends in the center console. When it's carrying 12 volts and the ignition switch is on, the passenger side fan runs. When the radiator coolant switch grounds and the ignition switch is on, both fans run. Seems overly complex if you don't have A/C, but probably cheaper to assemble the electrics one way. And of course no mention of this feature in any factory wiring diagram I can find. Thanks for the suggestions and photos - very helpful.

 

That has colors but they don’t bear resemblance to actual wire colors, they are just to differentiate between circuits. Actual colors varied over production - your best bet is to find someone with a build date near yours and get photos.