Pinto Car Club of America

I took my car to the shop to get the brakes checked out and noticed that while it was idling (it was dark so I had the lights on) that the head lights and also a light inside would get brighter and then dim and fluctuate up and down in brightness.  Is this an indication of an alternator on its way out?

Not necessarily. Do you notice when it fluctuates if the engine idle is fluctuating?

No it wasn't just based on engine idle.  I could notice fluctuations in the light output even driving it home.  While driving there were times when the lights would dim.  There is an aftermarket stereo installed although it was not on so I don't see how that would make a difference but perhaps I am wrong?

Double check both battery terminals and make sure they are secure. A bad ground can cause these symptoms. If you have an ohm meter you can run it to the battery terminals as well, and watch to see if your voltage is jumping around. If so it could be a bad voltage regulator (cheaper than an alternator). Most car parts stores can do a load test on your car to check both the alt' and voltage reg'.

Put a volt meter across the battery terminals while the fluctuating is occuring and post your results. 
Check the wiring at the voltage regulator - it has a tendancy to get bad connections and burn. 
Does the light switch get hot?  Will fiddling with it make the dimming stop? 
If you find that the alternator is on its way out, take the old one home and open it and check the brushes.  I replaced mine after 135k miles as they were worn.  Was easy to do, cost 5 bucks, and I keep the original alt!