Have you done the Ammeter bypass & added a new wire from the Alternator output directly to the battery?
If not it would be a good thing to do for the safety of the car & may solve the problem
Do you have any add on items that use power such as electric fans, fuel pump, stereo amps, halogen headlamps?
If so the OE style alternators don't put out much juice at idle so upgrading to a Denso alternator may be in order.
If the above are covered possibly there is something shorting out drawing extra current or your alternator may have issues.
The fact your lights brighten makes me think it is capable of some output but how much.. Your voltmeter (sounds like you have based on the #s you say tour seeing) should be showing more like 14V when the revs are up even with a lower output OE alternator.