You didn't state what year Impala you have but since we tend to ignore W-body (wrong wheel drive with a six cylinder sitting side saddle), and other newer cars that bear the Impala logo here I assume you do not have a one wire CS alternator on your car.
When your car was designed there were no power drains except for the wipers and the lights. The alternator (or Generator if you have a 1962 or older car) was sized to provide 36 Amps at 12 volts which matched the Wattage draw of the lights. If you are driving in the rain with the lights on the total draw exceeds the power out put and the battery will start to drain (the Gen light begins to glow). Power windows and other power draws were handled by engine vacuum back when your car was new.
This worked well for the intended purpose of the car (daily computer or family car traveling in day light most of the time). It doesn't sit well with our current 24/7 world with us in constant touch with everyone else and always on the move (life was simpler back then though having lived through it I would have argued with you about it had you asked me back in 1963 when I was building my first hot rod). If you install a modern MP3 player (and radio or CD player or what ever else it does) it will require more power than the total out put of your alternator to power those 48 inch sub woofers and 120 dB speaker system.
To meet modern power needs (electric fans, electric fuel pump, electric powered windows and seat, stereo, and most importantly a constant voltage for the on board computers since most cars have 14 to 18 separate computers monitoring voltage sensitive senders) the factory has increased the smallest rated alternator that will still bolt in place of the case first used in 1963 from 36 Amps to 107 Amps. Almost all CS one wire alternators are at least 120 Amps with many available up to 178 Amps in the same size case (you can go to a higher rated amperage such as used on police cars and medium duty trucks, but it requires you to change the mounting brackets because the case is bigger to hold more wire in side the case).
The factory went from a generator to a 10 DN externally regulated alternator to the 10 DN with an internal transistorized voltage regulator (avoid these) to the 10SI series which where all internally regulated and most out put 63 Amps or more (the identifying characteristic is the field wired changed from being side by side to being parallel to each other), to the new CS and newer alternators that have the fan on the back of the case for better cooling.
MAD Electric is a very informative site if you have any automotive electrical questions such as why do might lights dim when I slow down, and a cure to these issues.
Your best bet is to buy a one wire CS 120 Amp alternator and swap out the front pulley from a serpentine pulley over to your V-belt drive. It will then bolt up to your car. It is called a one wire as the only wire you need to connect is the now LARGER GAGE
red wire from the alternator to the battery. You jump from your stock ten gage red wire to an eight gauge red wire because that is the size required to move 120 Amps five feet. If you need a longer wire than that I would go to a six gage wire. You then disconnect the wires going to the mechanical voltage regulator. You will tie two wires off (wire nut or wrap in tape to isolate them) and tie the other two wires together.
This site sells CS alternators with V-belts already installed:
This sister Team site explains how to rewire the existing mechanical points so that your Gen light still works:
Oh I almost forgot the advantage of the conversion is no more dimming head lights or lack of available power for any electrical accessories. A CS one wire produces 80 AMPs at idle (your DN doesn't produce it's full rated 36 Amps of power out put until the motor is spinning at 3,600 RPM; which is why the lights dim at idle).