Fortunately, I found the root cause of my starting issue, but this may be useful info for someone later on.
First, the car:
1984 Pininfarina w/ Automatic Transmission (this is important!)
Next, the problem:
-I haven't had any real issue starting my car, to be honest. Well last week I decide to hop in the Spider and head to a Fiat Club event. What a great idea, right? I start it up, no issue, and stop at a gas station along the way. Once I top it up with a pleasant sub-$20 fill up, I get back in the car, turn the key, and.... NADA. Nothing! No click from the starter solenoid, no cranking of the starter motor, etc. Nothing!
Troubleshooting/Cursing/Road-side attempts:
-Fortunately this gas station had a large parking lot where I could move out of the way of the modern motorists who apparently give you 2 minutes after filling up before they honk and tell you to move.
1. The battery was not completely dead. I could put the key into accessory position and hear the fuel pump run as well as bright headlights. Make sure to make that a quick test and not drain what's left in the battery!
2. The ignition switch, to my knowledge at that point, should have worked. Or at least accessory worked and I had to assume the switch wasn't a problem.
3. Being an Automatic, roll starting wasn't really an option but we tried it for kicks and giggles. Nevertheless, no start.
I was frustrated at this point and decided to call the tow truck and continue troubleshooting at my leisure at home. That Antique Classic coverage from State Farm sure is nice.
Eureka at home:
So at this point I start troubleshooting the whole starting circuit.
1. Charged up the battery overnight to make sure I had full power. Still no start.
2. Voltage tested the Run position of the ignition switch. You should see 12.5+V (Battery level) across Brown wire (#30) and red wire (#50) when OFF. When you have an assistant turn the key, that voltage should drop to 0V. Anyhoo, the switch was good.
3. Is it the solenoid? This was fun/scary. I checked my manual on this and they recommended jumping the solenoid using a wire from the alternator power to the solenoid ignition input. Proceed with caution because even though it's 12V it's still nerveracking!! In any case, I gave it a quick jolt of power and the solenoid fired. Great! Now what?
4. I see in the relay diagram that there's an starter circuit cube relay. Hmmm... So I drop the relay panel and don't notice anything unusual. With no spare relays on hand, I noticed the power window relay was the exact same part number. I confirmed the power windows were working and then swapped the relays. With the starter relay now being used for the windows, I confirmed my relay was good.
5. The dreaded C31 connector issue. This is the block connector directly behind the ignition switch which can have all sorts of problems. I didn't have the "click of death" symptom, no clicking at all, actually, but I proceeded to check the connector. Nothing doing.
FINALLY, there I am lying on my back on the driver's seat looking up at the god-forsaken nest of wires under the dash and there it is. A simple unplugged Violet/Black wire that used to be a ground. The vibration of the road and a loose fitment had finally taken it's toll and my ground wire for the Automatic Trans-specific starter & back-up switch came unplugged.
With it back in place and crimped more securely, VROOOM, she fires right up.
I must say the starting time has even improved slightly due to the better ground and what I assume is more/proper current level reaching the starter. It now fires in 1sec instead of 2-3.
Now we're back in business!!
Executive summary: It's always a d**n ground
