Why would my battery drain faster than it can charge when using my lights and wipers

[Misstere]

We have a '61 Gmc 1500 pick-up. It runs great and we just bought a new battery for it. Driving home at night with the lights on will drain the battery quickly and using the wipers with the lights is not an option.

Can anyone tell me if it is most likely the regulator or the generator and why? Is there a way I can narrow it down?

Thanks so much for your help.

Jeannie B.
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