i got this problem with my car...
everything dims when the engine is idling, above 1,000 RPM everything goes bright again. Even the indicator when its on and you're just idling its very slow or sometimes just stops blinking its just on.
I got a amp gauge and its reading below 0 and it reads above 0 when car is moving or above 1,000 rpm.

I checked fuses, fusible link and alternator is charging 13.5V
battery is quite new few months old and tested it for 12 - 12.5 V
I opened the alternator to check brush and probably got 8mm - 10mm from the limit. Cleaned the inside as well and still the same.
Check the fan belt delfection, timing, change spark plugs, points, and rotor button.

do you think its just the alternator?
I heard it can be the ground?

I would suspect its the ground, but you might put a trickle charger on the battery over night to make sure its got a nice full charge. Do you drive short distances usually or take it on long trips? A lot of short hops at night can reduce your battery charge.

I was having the same problem even with a new problem so definitely not the battery.

This problem is actually for my other car 240K
L24 engine

Anyone know which cars alternator I can take to bolt-on to my 240K?
heard EA falcons for 1200 dont know about 240Ks, are 280ZX or maxima straight bolt-on job? i know wiring will be a little different....
I would prefer high capacity alternator
80 + amp