I can tell you how to do this, but it will not fix your dimming problem. If anything it will be a hindrance as your already strained alternator will have to charge two batteries instead of one. The only reason I would add another battery is to allow me to play the stereo louder when 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)