Frickin fiat!!!

Similar experience with Mustang convertible and this goes back to 90s models. New battery, starter spins great. Odd as it seemed, at switch on no relays clicked. Should have lit up the computer which in turn would cycle the fuel pump for 3 seconds. getting none of that. Add a 50 amp booster box to this new battery and bam, everything clicks, she fires right up. This worked for about half dozen restarts, then total failure.
Fault found was the tiny brush bristle like terminals at the computer to harness connection. Only a white powdery ash like residue remained. Evidently the verts get a little more moisture in the cabin than others.
Chrysler had about the same problem in their minivans when they placed a similar connection outside under the cowl.
If there's anything good to say about our classics, water could pour from the dash and drain right on out through the rusted floor, while we keep on going and going ( place pink rabbit here ). Good luck with it.