From an auto electrical site:
A possible problem with a warning light is that it goes out as normal when you rev the engine slightly but comes on again as the engine speed is further increased.
If the warning light simply glows dimly it usually indicates the problem is a high resistance somewhere in the warning light circuit.
Test this by switching on more and more electrical accessories once the light has started to glow. If high resistance is the problem, the warning light will become brighter as you do so.
Carefully work your way through the entire charging system looking for loose or dirty wiring connections causing the high resistance. Disconnect the wires and clean up the terminals if they are dirty. Don't forget to check whether the battery lead connections and engine ground strap are still good.
Remember also to check the alternator to engine mounting bolts or strap. These provide a ground passage from the generator to the engine and body, but because of their exposed position in the engine bay they can become oily and provide only a poor ground connection.
Check this connection by removing the mounting bolts or strap. Clean away all traces of oil and dirt then refit the mount, making sure they are tight.