that tells you your wires were corroded so you fixed that part. Its just likely your pulling too much from the alternator. my lights dim too when i bast my stereo, my system runs at at least 1000w rms when I crank it up, my 140amp alternator barely keeps up at a reasonable sound level. I try not to keep the music at the point where my lights dim because it is bad for the alternator, the rectifier diodes in it heat up when there is excessive current draw and will eventually fail.
It could be other things like the battery but most likely its just you pulling to much amperage. either lessen the load like how I mentioned in a previous post or increase your power supply (alternator).
lay off of the volume when your truck is idling as the alt does not produce its rated power at that point only when your engine is at a higher rpm does the alternator produce its rated output.
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