Electrical frustration with 1991 e-350!

was going down road and went to turn on a/c,van lost some power,gauges went everywhere and radio cut out.was able to make it home and also saw that when headlights came on van would stutter and shut off.it seemed to struggle everytime i wanted to put any kind of load on it,so i figured its electrical problem somewhere.charged up battery,which was kinda drained,replaced alternator with a known good one,replaced voltage regulator(external-which surprised most auto parts places i called)belt is in good shape,replaced both battery cables.i couldnt put the ground to chassis bracket on cause cable was a little too short so i attached a ground up by the battery to the hood latch area(metal).i replaced all this one by one to whittle down the symptoms.now my van turns on well unless i leave headlights on too long with engine off(battery doesnt last too long,i know i need a battery soon).i tested alternator and tester says its charging,14.something volts.checked for power at alternator with engine off and there is power,read something about the fusible link being bad if there wasnt power there.problem is,the other day we had the radio back somehow and turn signals,now we dont have either.i also dont have power to the turn signal relay at fuse box,by the way,all fuses checked out.i checked rear brake lights(good)rear tail lights(good)but when these are on and i turn on the blinkers,one or the other will cut out at the rear.battery gauge also has a little flutter too it,its faint but i notice it.the only thing i did different was that battery to body ground,so i went back and took it off but theres no change in the symptoms!any and all advice welcome,no offense to any garage owners here but i like to go over everything i can before i give up and take my cars in,we've been taken a time or two.id also like to check the codes but i think i did it wrong can anyone show me a picture of where to put the jumper wire and where to put the test light?i dont have the actual code reader but read somewhere you can do it like this.any and all help,please!