On that car there is a possibiltiy that poor ground connection with the wire from block to back of alternator (Blue wire) could be loose or dirty. The more likely problem is that one of the non-serviceable diodes that rectify AC into DC is open or shorted and has effectively reduced your alternator capacity by 1/3. The alternator warning lamp works by tapping into an internal supply current that is used to power the voltage regulator. When the alt. is charging the voltage through the ignition switch to one side of the bulb is just slightly less than the internal alt. voltage and the bulb doesn't have enough current flow to glow visibly. When you turn on ign. switch but before starting engine the light glows bright due to a circuitous path that goes through the alt. ultimately to ground, thus grounding the bulb and making it light up. When the diode(s) fail the voltage from that internal source may be 18V and the voltage from the ign. switch to the bulb may only be 12.8V and you get a dim bulb, the changing brightness follows the demands you put on the electrical system, i.e. the signals flash and the ign. sw. voltage falls to 11.9V and the bulb gets brighter. The reason that the internal voltage is so high is that the regulator is giving full current to the brushes trying to make the weakened alternator works its little electronic heart out trying to keep up and the result is increased voltage before the output rectifying diodes where the alt. light terminal is tapped in. What does all this mean? To do a crude test for the faulty diodes take a reading with a volt meter between the two terminals on the back of your alt. with the engine running at a fast idle (1200 RPM), the reading should be something no more than one volt. The real way to test is with an oscilloscope reading the small terminal with small gauge red wire to ground and checking for a smooth ocean wave signal as opposed to ocean waves during a hurricane (hurricane waves=faulty diodes)