WTF

Steve, offhand I would say the black wire at your connector is the wire from the wiper of the OP sender that goes to your OP gauge. I'm guessing the wire for the oil pressure lamp doesn't come into this connector and goes somewhere else.

To confirm which wire in that connector is for the OP gauge:

1. First, put your meter on resistance, then earth one lead to the chassis, or the battery as finnerty suggested, and now measure the resistance of all the terminals on that connector you showed in your photo and write them down. Leave the oil pressure sender connected as you have it in your photo. I'm guessing you should have about 10-12 ohms to earth on that black wire.

2. Now, get a close look at the backside of the oil pressure sender where the wires terminate and see which terminal is marked "G" (Gauge) and which is "WK" (Warning Kontact). See my pic below, which is of an old 550 oil pressure sender, and which should be identical to yours. I'm guessing, based on the position of the "rib" on your sender, that the wire with the yellow marker is the one you want.

3. Disconnect the wire that is connected to the "G" terminal.

4. Now measure the resistance of all the wires in that connector again, and see which one changed - probably the black one now reads infinity? That should be the other end of the black wire you disconnected at the sender.

I would guess the other wire on the sender, connected to the WK terminal, does not go to the connector in your picture, but perhaps to another connector for dash warning lights, etc. Either that or Ferrari changed the wire color between the sender and the connector!
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Steve ---

I don't know what spec your car is, nor do I have detailed knowledge of the 355 electrical system. But, in general, on any car made with different variants and model year revisions, it would not be unusual to have unoccupied fuse / relay positions ---- depending upon year, US / Euro, spider / berlinetta, 6-speed / F1, options installed, etc.

So, I doubt that you are actually "missing" any fuses / relays ---- they are most likely simply not used (and were never installed) for your particular car.

Your owners manual should indicate what those locations are used for ---- therefore, you can determine why your car does not need them. Also, very often a fuse / relay panel in a more modern car has empty locations simply because the manufacturer sourced a panel in a generic configuration from the supplier with no need / intention to ever fully populate all the available locations. Come to think of it, I cannot recall ever seeing a completely occupied panel on any (make or model) car I've looked at since about 1985 .

 

Here's what the parts manual shows for that panel ---- passenger side location on an M5.2 355 ----
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