240D battery light on at idle
 

My wife called and said the battery light is on or flickering at idle, but goes off when she drives the car.
The last time something like this happened to me was with our Olds - I think the light was staying on all the time. I drove it to a shop and paid a guy to diagnose it. It was a bad alternator, which I replaced myself (found a real good deal on a NAPA reman unit).
I know that the belt is suspect if the light comes on in the rain or at high revs or accelerations.
Any easy diagnostic checks I can do?
Or should I just go ahead and replace the alternator?
Thanks.
Ted

240d battery light
 

I had this happen to me and it turned out to be a worn voltage regulator which can be changed for about $30-$35. The brushes on the voltage regualtor are usually what wears out.