i usually take my rearviw mirror off. it's easier that way for me. i've had mirrors fall off and windshields split in my cars.
about the dimming and fluctuation of rpm and voltage:
your best bet is to upgrade the stock alternator. it will only put out up to 90 amps cold. when your system is on, it fights your vehicles electical system for power (lights, heater fan, A/C, windows, power steering, power brakes, spark going to engine, etc....). a 130 amp alternator will help, but a 200 amp alternator is definately worth the extra $$. check out mr. alternator, mean green, or if you've got money to burn... Stinger.
also how thick are your power wires? 4 guage would be best for your setup.
if you dont have the cash for a new alternator right now, then maybe you should check the voltage regulator. i dont think it's going to be the answer, but it's possible if you get a lot of fluctuation.
a 1 farad cap will help with intense deep bass hits when the amp gets starved for power. i use capacitors in my systems and the results are great. but i dont think it will do much to help your current situation. they store power for quick use, but after an extended period they need to recharge. and they take a lot longer to recharge than discharge. that's why you should get the alternator first.