I need y'alls help

[Fire_boy3]

Hey y'all I need some help. I got a 65 gmc c10 that had a 350 from a 84 c10 dropped in it. Well about a week ago I went to leave my buddies it wouldn't start so I jumped it then the next morning it happened to me again so I got a new battery hoping that would fix it. Well it didn't so I took it to get the alternator tested and it wasn't charging enough so I got it replaced and put a new one in it worked for like a day and it happened again so I got that one tested and it didn't work so I replaced it and put the current one in. I got it tested that day and it's working charging at 12.6. But earlier tonight I went to start and lo and behold the battery was dead. So it's not the battery or the alternator so I must have something draining the battery well the day before the problem started I started having issues with my right turn signal and break light (my grandpa added another break light to each side) so I got down there and found a bare wire taped it up but that didn't fix it. Also about 3 days ago I turned a corner and my radio quit working so earlier tonight I checked the fuse box (non original fuse box) and found the radio fuse melted. Could that melted fuse cause the battery drain and do y'all have any idea what could be causing the problems with the break light and turn signal? Could the turn signal and break lights be what's causing it? Me and my grandpa have done everything we can think of to fix the issues but nothing worked. Sorry for the wall of text but I'm just at a loss right now.

[PGSigns]

Welcome to the board and nice to see you are working with your Grandpa. Well 12.6 is not a good charge rate, 13.6 would be a better number. 12.6 may be just the battery level. There is a voltage requlator on the drivers side of the radiator support. That may be the charging issue. There should be a wire from the alternator to the horn relay and from the horn relay to the battery, make sure it is in good shape. With the wiring in the condition you have described it is going to be hard to help with the battery drain. Most light issues with these trucks are bad grounds. The first thing I would to for the lights is ad a ground wire from the bed to the frame and from the lights to the bed. I have had to do this on several of mine to get lights working well. There are wiring diagrams here (FAQ section is a great place to start) for these trucks and one of those and a test light will help you follow stuff out. Post pictures of what you are working with and we will take a look and see what else may be giving you trouble.
Jimmy

[old cruiser]

I agree 12.6v isn't enough should be 13-14v.
Also a quick check to see if something is draining the battery is to loosen earth cable from battery and remove slowly if you see or hear any arcing as you remove or refit the cable there is something drawing current. note make sure nothing is turned on when you so this.
I f you do notice some arcing you can then one at a time remove each fuse to isolate which cicuit is causing the problem.

[Fire_boy3]

I apologise I meant to put 13.6 long day fried my brain. Thank y'all for the help I'll try it and get back to yall.