With everything dimming and/or running slower when you turn on more stuff, what Garret and others posted about checking power supply is good place to begin.
What does the ammeter show when these items are turned on?
Also if you have a voltmeter, check the voltage at the alternator and then downstream where ever you can access, such as battery positive, cigar lighter or at the lamps.
That or power supply is limited because the battery is low or the alternator is not producing at 14 Volts.
I'm suggesting make sure the battery is fully charged and the alternator is producing power. Hence checking the ammeter to see if the battery is supplying power when the brake lights are on with the other stuff, as well as checking voltage.
