It may be a weak/bad "Exciter diode" in the alternator—not a brush/regulator problem, especially if he lights glow brighter as speed increases.
Here's why...

With the engine running, the output from 3 small "Exciter Diodes" goes to the D+ (small red wire) terminal, and from there to one side of the warning light.

Three larger "Positive" diodes make the B+ output that goes to Battery, and from there to all car circuits — including the other side of the warning light. Since each diode trio should supply the same voltage, that warning light should be OFF — because with the same voltage applied to each side of the bulb there will be no current flow.

But if the D+ voltage is lower than B+, the unequal voltages will allow some current flow thru the bulb. And if the B+ voltage increases but D+ does not, the light can glow brighter as speed increases — as has been reported here in the past.

With the engine running, compare the D+ and B+ voltages. If D+ is lower, a defective exciter diode wire is a good possibility.
--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.