My son has a 1995 Toyota Avalon with over 100K miles. He recently had the engine replaced with an engine with 30k on it. The car ran fine up until about a week ago when he notice that when, different interior lights are on and the brake pedal is pressed the lights (interior, dome) become dim.. As more electrical things are turned on the dimmer the lights become. The passenger was reading with the dome light on that would dim every time he would step on the brake. When he turned on his head lights all the lights went out (headlights, dash, dome, everything) No effect on the engine, no loss of power. As soon as he turned the headlights off the dash lights came back on slowly. He went to AutoZone and had the battery and alternator checked, they found no problem. Battery cables clean and tight, Alternator connection appear to be ok. While he was at AutoZone the sales person started the car and turned on the headlights and the car appeared to work fine.

I think that there is probably some kind of short there, or one of the cells in the battery might be close to dry.