"Those three lights stay very dim after the car has been started."

Let's go back to those lights before starting. Earlier you said they were dim when you turned the Key on. If they aren't as bright as the other lights (e.g., Oil warning), that's wrong and must be analyzed.

Try it again, and if those bulbs (4 actually, counting Bulb Failure) are dim, take the small red wire off and ground it to the block. If the lights get brighter, the problem is in the alternator, regulator, brushed, or blue ground wire.

Test the blue ground wire by grounding the small red to the alternator case. If bulbs get brighter, the blue ground was not making them dim, but those other items are still questionable.

If they stay dim, see what the voltage is on that disconnected red wire. It should be the Battery voltage, feeding thru those bulbs. And yes, there is a connector between the cluster lights and the alternator -- it's gray plastic, on the right/center of the firewall (8-pin but only 4 or 5 wires, including our red one). See stolen pic (ignore arrows).

Here's how those bulbs should work...

• The Battery light (and 3 more) come on with Key On.
• That's due to battery voltage on one side of the bulbs — and ground on the other (from cluster, thru small red wire to the inert alternator D+ terminal).
• The lights go out when the alternator ground path (D+ small red wire) rises to alternator output voltage (B+ big red wire)...
• ...because with voltage now on both sides of the bulbs, there's no current flow — and thus the lights go out.
• If the lights glow when running, the alternator voltage at the D+ red wire must be lower than "battery voltage" (now at Alternator B+ level).
• This difference in "potential" allows a small current flow—and thus the glowing lights.

Most likely cause is at the alternator, assuming the lights are bright with Key On.

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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.