I wouldn't spend the time trying to replace a diode myself. Not worth the effort. I'd take the alternator to a local electrical shop and have them diagnose the problem and at the same time service it; basically the same suggestion as John. Once you go to the effort of removing the alternator, you might as well replace the brushes and if need be the bearings as well. I paid $99 for this job at one time.
Alternately (play on words here), you can buy a rebuilt unit for a couple hundred dollars and be done with it. However, before you do, it's still worth taking it to an electrical shop to have them test it under load and confirm it's bad. It is feasible that it is something other than the alternator but your description of a bulb that changes in brightness does point toward the charging system.
The bulb you mention works on a differential between the alternator charging voltage and the battery voltage. If the alternator is not charging, there will be a differential and the bulb will light.
Thie link I've attached is from the Triumph board but provides one of the better explainations I've seen on how alternators work.
I've replaced mine 3 times in the 21 years I've owned my car and each time I also replaced the voltage regulator as well. I don't think it's mandatory but I do it just in case whatever damaged the alternator also affected the VR.
I won't try to diagnose an alternator problem over the internet as there are several things that can cause it not to charge. As mentioned, one is a bad diode. A second is a broken wire to the brushes; and a third is a broken hold down spring that keeps the brush in place. Ask me how I know.
One other thing, put your battery on a charger if you have one so it will be at full capacity when you reinstall your alternator. It may be worth a trip to a local Sears to have them test your battery and recharge it as well if you have any doubt whatsoever that your battery is good. I like to eliminate as many possibilities as I can when troubleshooting electrical issues.
http://www.vtr.org/maintain/alternator-overview.shtml