When I am coming to a stop from 40MPH during the shift from 4th to 3rd my headlights dim for a split second as the gear changes and Rpms drop. Would this be a transmission problem or something electrical like a bad ground? I was thinking of adding a 12 inch sub soon but with this going on I don't want to make it worse. Not really a problem just kinda annoying thing I been noticing. I believe it only happens when downshifting from 4th to 3rd. I do not notice anything in the interior lights having any dimming just the headlights. If anyone knows what could be causing this it would be a great help.
Car is 2004 SE 5 speed auto with 142k miles.

 

You might want to check your alternator. And yes, make sure your grounds are making good contact.

 

You're alternator is about to go out. Go have it checked asap before you get stranded.

 

YEP! this happened to a buddy of mine a couple of days ago, he actually needed a new alternator belt it was cracked and slipping, by the time they pulled it off and checked it the pulley on the alternator was crapped and had eaten the belt.

 

+1 alternator. Although it is wierd that it only happens when shifting from 4th to 3rd. Get it checked out...

JB