First thing first, have your battery load tested. Your battery may be at the end of its life. This causes the alternator to have to pour more current into it in order to charge it, which leaves less for the rest of the system.
The dimming is due to voltage drop caused when the alternators max output is reached, the battery then kicks in and at its slightly less voltage to pick up the slack.
I'm pretty sure everyone here is aware that the alternator powers the truck when it is running.
The battery charges and provides no power until the load becomes too great for the alt. at this point the alternator drops its voltage in favor of providing more current. If the voltage drops down to the battery's resting charge level then the current will begin flowing from the battery, meanwhile the alternator is being stressed trying to keep up with the system load and trying to keep the battery charged at the same time. This causes the rectifier diodes and voltage regulator in the alternator to heat up and eventually fail.
The best option is to beef up the alternator.
the "big 3" should be completed before installing the HO alt. due to the higher current potential. otherwise the big 3 is basically a patch job for corroded ground cables.
Your other option is to buy a bunch of batteries and pray that you have enough a residual charge leftover at the higher voltage to get you through to the next music peak not likely to happen and can be very costly.
Some things you can do in the mean time while your waiting for the alt or battery.
-Turn the music/bass down while idling. this will prolong the life of your alt and batt.
-Use a subsonic filter if you have one, turn it up to the point where you can just barely hear the effects from it, then back it down just slightly. this will prevent your amp from wasting precious current on inaudible tones.
-Relax the bass boost some if your using it. its the biggest current hog in the system.
Toreador Red 05' FX4
4.0 4x4 A/Tran Ext.Cab Daily Driver. Nomads, BFG M/T's
Alpine 9857, JBL MS-8 DSP, US Acoustics 2x80w & 2x100w, Vifa 1" XT25 Dual Ring Radiator Tweeters 3k+ 24db slope, Dayton Audio 7" RS180-4 Mid-woofers 100Hz-3k 24db slope, 500w rms Kenwood Class-D Mono - JBL 10" GTO1014 Sealed. 100Hz- 12db slope