You might make sure your battery cables are in good condition. That is the most common problem with power systems on old trucks IMHO when the inside gets coroded they dont hold good current causing the batteries to not get a good charge

 

i have brand new cables and it still dont work

 

X2 on the regulator being out, mine did the same thing a while back and I just ran one external and it works just fine and has been for about 3 years now