HELP! Dimming lights question...

I'm having issues with my 2003 Impala. Just recently the lights starting dimming when I was in park and/or braking - I had the battery replaced because it was near dead and then the car did the same thing AND it started jerking when the car was shifting gears(it's an automatic). I brought it back to the mechanic and they ran tests and determined that it was the alternator(even though the first test showed nothing wrong with it). They replaced the alternator and ended up having to replace the battery again. Just drove home and the car is doing the same thing. Any ideas? I do have satellite radio installed in the car,but have had it for a year with no issues.

Any help would be greatly appreciated!

I would check grounding connections at both ends might be loose or corroded.

How bad is it? Mine will dim when i am stoped and useing something like the heated seats or rolling up a window, or useing rear defrost. It is some what normal to dim your lights a little. There is a fix i have seen somewhere when you put something on the wire that comes of of the alt.

It's pretty bad actually. All the lights(interior and exterior) dim and then go back to normal and it happens over and over again. The car also feels like it is about to die out when the lights go dim. I did a search now that I had more time and found some past threads on it being possibly the bcm/pcm(?) I asked my mechanic and he said that he checked that yesterday and it was fine. I'm really lost at this point...especially since I've paid $500 in the past week for two batteries and 1 alternator and the problem is still there.

It's pretty bad actually. All the lights(interior and exterior) dim and then go back to normal and it happens over and over again. The car also feels like it is about to die out when the lights go dim. I did a search now that I had more time and found some past threads on it being possibly the bcm/pcm(?) I asked my mechanic and he said that he checked that yesterday and it was fine. I'm really lost at this point...especially since I've paid $500 in the past week for two batteries and 1 alternator and the problem is still there.
Pretty hard to diagnose without voltage numbers ie: alt voltage output (should be around 14.5) Batt should read a steady 12+ volts...Any aftermarket junk been put in like a radio or amp, lights? Sometimes corrosion can't easily be seen could be in cable from batt, it can work it's way down through the cable or a bad cable end. Starter ground could be loose. Serpentine belt tensioner may be going bad putting a drag on system. Recheck alt connections. I would also mention I have heard of getting a bad new alt in the past but a voltage check will determin that.