Dim dash lights

Thanks for any help. I have a 65 Coronet and a 67 Barracuda. Both have dim dash lights. I Have installed new LED bulbs, new headlight switch, checked the bulk head dis-connect (cleaned) ran a ground wire from the dash to the - battery post, and checked the disconnect on the printed circuit board on the back of the dash. Bulbs are still dim. Led helped a bit, but still very dim. HELP.
What else can I do? I did this to one car the 67.

Measure the voltage right at the bulb. You might be getting a voltage drop due to old/bad wiring.

There's likely 40+ years of dust in those instrument panels. Make sure the green domes that the bulb inserts into are clean inside and out. The layer of dust could be reducing your light output.