Generally the dynamo control box is mounted on the front wing not far from the dynamo. But it may actually be on the dynamo or on the bulkhead, or even at the back of the dashboard. Many control boxes have resistors mounted on the baseplate.
On cars with dynamos, faults in the control box show up in various ways, some of which are described in Testing a dynamo and checking output.
Others may be revealed by the ignition warning lamp. If the lamp does not go out as the engine speed rises, the cutout points are not closing.
If the lamp glows dimly at low speeds and brightens as speed increases, the fault is probably dirty, oxidised or burned cutout points.
If the lamp goes out at an abnormally high (or low) speed, it may be a sign of the same trouble in the voltage regulator points, or that either set of points is wrongly adjusted.
The points are tricky to deal with, and the job should be entrusted to an auto-electrician.