Why not cook marshmallows on my under dash fire?

I can't tell, frankly, if the "big splice" (the original welded one" is adequate or not

First, I think yours was OK, but too late now

The soldering looks cold. This "old wire" can be a challenge, as it can be heat damaged and oxidized.

Sometimes what helps is to "fray" the strands out GENTLY and GENTLY scrape them with a knife / razor, to get the copper clean.

That splice is BIG and should have been twisted, and soldered with a LOT of heat. The soldering looks "cold." (Not good.)

In this particular case a better method might be to get a 10GA "butt splice" clean the wires as noted above, and crimp them in the splice, THEN solder the whole thing

What, exactly, are you using for an iron or gun? Size of tip and wattage?