Electrical Chaos with 365 GT4 2+2

When I bought my car a few months ago, every little electrical device in my car was working perfectly. The car itself had been standing for a while which was obvious because the tires had a little ding in it. But again, everything worked - AC, fans, interior lights, gauges, even the light in the ashtray worked. I drove the car for about 2000 km and nothing ever failed to work.
Recently I put my car into the shop because I wanted the entire suspension and steering overhauled. Also, they put in a Dinoplex conversion (voltage booster) which ferraripartsexchange send me instead of the electronic ignition conversion that I had originally ordered from them. Furthermore they exchanged the window washer pump for a new nonoriginal one.
After I picked up the car again I gradually realized that my entire electrical system is screwed up: The main beam sometimes works, high beam never. AC sometimes works. Heater fans and heated rear window don't work. The radiator cooling fans sometimes work (of course, same with AC, never when it's really hot and your standing in traffic...). The clock has stopped working completely. Even the new washer pump only sometimes works. Also, when I turn on the lights, all gauges go up 5mm in their reading, except the gas gauge which goes to completely full no matter how full the tank is.
I called them up and told them about this, and the response was that when a car was standing for a while and then you suddenly start using it again, devices will start to fail - because of that sudden usage...
I think this is major nonsense. First, nothing - absolutely nothing failed while I was driving the car before it got to the shop. Secondly, non of these failure came gradually, one after the other - they were all there after I picked up the car. It just took me a while to realize some of them.
All the fuses are new and fine and the fusebox and its surroundings look like they're in perfect shape. I doubt it's any of the relays since the problems are spread over too many circuits.
Does anybody have an idea what this might be..?

Thanks,

Howard