I've made so many posts about the BROWN WIRE FIX over the years that It's way over due for a FAQ on the topic. Between bad grounds and the brown wire fix, it seems that these two issues are over half of the electrical problems associated with spiders. examination of the article here and the ignition switch article will give you the information you will need to eliminate this annoying and potentially dangerous situation.
One of the bottlenecks in the Fiat spider electrical system is that regardless of the year of the car, all of the 12v electricity that the car needs gets funneled through a single terminal in the ignition switch.
Back in the day when this amounted to the lights, wipers, heater fan, and perhaps a small radio things went well, but in today's cars with modifications and also due to a switch in how Fiat wired the ignition switch with connectors and spade push on terminals sometime around 72/73 certain issues have come up over the years that have caused complaints about dim headlights, slow wipers, dim dash lights, and all sorts of other issues.
Required reading for this article should include an excellent article by Chris Coulter in our FAQ section with some diagrams and switch position explanations that will go a long way in helping you to understand how the ignition switch operates and also will give you the background you might find useful about how the brown wire thing has developed. Check out the picture on the right to see the wire colors and the diagram of the connector behind the switch. It also gives you a pretty good idea of what the different switch positions do. You can see the articles (don't forget to follow the links at the top of the article also) HERE.
First and foremost, you need to understand how the spiders get their main supply of 12v. Regardless of the year, the power from the battery is brought up to the starter solenoid stud. Briefly, the brown wire connection goes from there up through the firewall and depending on your year car, either directly to the back of the switch or through a white plastic connecter to the switch. This white plastic connecter and the stud on the starter solenoid are two of the main reasons for voltage drops in the system.
The brown wire fix can be approached in one of two ways.
1. The Quck Fix:
A good cleaning will help things get back to normal, but you will still have a marginal system. I strongly recommend that if you have the white connector, you remove it and solder the wires together or at least use butt crimp connectors to eliminate this source of resistance. I also strongly recommend that if you have one of the later cars with spade connectors on the back of the ignition switch that you solder the terminals as shown in Chris's article. You run a much smaller risk of being left in the dark some rainy/snowy night when one of the terminals decides to quit working.
These procedures will greatly improve the performance of everything connected to the battery but will not long term solve your problem.
2. The recommended solution:
Recommendation: One of the best things you can do for your ignition switch is to check our FAQ on using a simple relay for your headlights. This will go a long way to brightening up both your headlights and your dash lights, and also take a lot of the heat off of the headlight switch on the dash and the steering column.