Hope This Helps
There is some good info here, I had a some volt problems in my X.
I hope this helps, I agree with Cobraguy that you should be looking for volt drop. you have a multimeter so here is where to start ( I could not get a clear pic of the back of my alt. in the car so here is one that out of the car).
Ok now wright down voltage from the back of alt. this is your reference point.(with the car running

) them check at the battery, them I would also test at any new buss location you have added with the aux thing you have added turned on. record the voltage at each location. if you do not have volt drop at battery but you do at your aux power location than you need a bigger feed wire for you aux.
Grounding...Grounding ok so in my system I installed a 240 alt. 2nd battery, 0 gauge wire every were and was still get volt drop bad. the dash volt meter was so low any time I ran my stereo, lights, A/C, fried my front battery and had a hair pulling time with how to fix it. so were I started was running a o gauge ground right from the back of the alt. to the frame ( also used 3/4" heater hose around it to help with heat) 2 gauge from alt. to front battery, 0 gauge to back battery, 4 gauge from front battery to input of main fuse buss under hood (note all power had been done and still had volt drop the only thing I added in this testing part was the ground)
then I added a 0 gauge ground from frame to back battery. retesting after frame ground all volts were stable from front to back even with the stereo lights a/c all on at the same time.
so if you have volt drop from alt. to battery I would not trust the ground from the engine do a frame ground from alt. then a frame ground to battery.
if that does not fix it I would try adding to the + off of alt. to battery do not remove stock wire just add to it unless you now you have a sort in it. I would also recommend running a better ground from frame to the chassis, most thing in the car will pull a chassis ground and I find it is not a good ground, once you have a good frame ground you can always added to the chassis were you need it.
I was so supprised that with all the power and the gauge of wire that my problem was just grounding.
the problem with volt drop is that anything in you elec. system works at a given voltage and draws a given amp at that voltage if you have volt drop it takes more amps to do the same job more amps means more Heat and heat is what will kill your system and the components in it (ie you battery alt lights fan blower) all of this is affected, so big stereo or not volt drop from bad grounding is probably the best thing to fix for the elec. system and its not that hard to do.
well now I feel just like I am rambling on so hope this helps and PM me if you have ? I have spent allot of time with the power in gen 2 X's