Those guys don't know what they're talking about. A battery's primary job is to start your car. After it does this, it becomes a load on the alternator. Your car's electrical system draws current from the battery with or without the car on. When the car is on, however, the alternator keeps the battery charged. The dimming occurs when the alternator is not able to supply enough current to keep the voltage up in the battery.
So, conseqently, an additional battery would do nothing, and it might only make it worse. The only good an additional battery would do in your case is allow you to play your stereo longer while the ignition is off.
1992 Toyota 4Runner SR5 4WD (5 Spd.)
Pioneer DEH-P6500 Head Unit
Boston Acoustics ProSeries 4.5 Components (Front)
Boston Acoustics RX47s (Rear)
JBL P-80.4 Amp (for front's and rear's)
Boston Acoustics ProSeries 12.5LF 12" subwoofer
Alpine MRP-M200 Power Amp (for subwoofer)