I found these to be helpful in my sluething out the most common Fiat Spider wiring issues.These diagrams are subdivided into eight pages broken down by function for simplicity. Although these diagrams are for the 1976 spider other years are similar. I've found the 1971 and 73 cars to be very similar for many of the systems without relays.

These JPEGs are scans from the shop manual for the 1976 car. It's probably best to download and print out the wiring key before trying to decipher the diagrams.

Diagram 1 Relay Locations
Diagram 2 Wiring Key
Diagram 3 Alternator Circuit
Diagram 4 Fuse A Circuit Wipers
Diagram 5 Lights
Diagram 6 More Lights
Diagram 7 Gages
Diagram 8 Ignition/Coil Circuits