New to forums, new to Mustangs

Hi and welcome, does the light go out with a little increase in RPM? There are a couple of possible causes. The old mechanical regulators are not the best, you can get a direct replacement solid state regulator that's much better at the job, and if you want the old look, change the thin cover for the old thick cover. You could have lost one or two of the diodes in the alternator, it will still work kind of but can cause your symptom, a load test will show that problem. if you mustang is early with a generator, it could be normal, but again the light should go out at higher RPM's. Clean solid electrical connections are a must, check this first. Good luck.

Could be your alternator is getting weak or you have poor connections or really old battery cables. They deteriorate over time and get corrosion build up which doesnt allow them to carry enough current. Most parts stores will test your alternator for free. Then you should check and clean your vable connections and check your alternator connections on the back of the alternator.

Check your battery voltage when the alternator light is flashing. It should be in the mid to upper 13v range when the engine is idling. If that checks out, then I agree that it's probably something going on with the indicator light circuit rather than the charging circuit.

I didn't get my license in the '65 Mustang I once had, but it was my first car. And it was white.