John is on the ball tonight with an excellent picture and he's correct on all counts. The general schematics don't cover accessories like power windows, power seats, convertible top, sequential signals, etc. (if your car has any of these).
1955 was the last year for six volt systems on Ford cars. I have one.
The large box mounted to the firewall is the relay and the small box is a circuit breaker. CAUTION: Yellow wires are UNfused battery power (+12) coming from the starter relay. At the mentioned circuit breaker, overcurrent protected power feeds the relay contacts. The relay coil is energized from the key switch. Sometimes this relay is used to feed the convertible top motor as well.
Important to note; the relay box needs a good ground, much the same as your starter relay, or it won't energize. So, make sure they are tightly bolted down with no rust inhibiting 12 volts.
If you have more questions, please ask. - Dave