Buy a Weller!..you'll only buy it ONCE..
I'm still using the Weller soldering gun my DAD bought before I was born probably!--in fact,I just used it yesterday to solder a new dimmer switch harness in my 82 K20,so my lights might actually stay on at night when I click the dimmer switch from high to low beams!---and I also added another "hi beam" indicator lamp to my dash,because I could not get the one in the speedo cluster to work, due to a printed curcuit problem,and I didn't feel like tearing the dash out..I got warned by a cop to shut off my high beams twice in a few days,so I figured I'd better do something!..
As for soldering wires,I find the biggest problem is lack of enough heat,and the type of flux used...don't use paste flux designed for copper tubing,it will corrode the wires later on,and it doesn't really work all that great on wires anyway..
I use rosin core solder for wires,do not use "acid core" solder that is used on pipes and radiators either--it'll solder the wires all right--then corrode them away in short order!..if I need more flux than the solder has in it,I use a clear liquid flux called "tinners" flux..it's more for tubing or radiators too,but you only need one drop to get a stubborn wire to take solder..it doesn't seem to corrode them too bad later,and I've even been able to solder galvanized steel together with that flux and acid core solder,fixed a galvanized bucket that had some holes in it...usually steel is a SOB to solder,but this flux made it pretty easy!..
I find soldering old GM wiring harnesses a pain..usually the wires are loaded with corrosion,all green or black inside..you MUST scrape that crud off before soldering,or it'll never stick,even with flux!..also,I think many of the wires are not all copper--some look like stainless steel inside!..those are the hardest ones to solder,using crimp connectors might work better than solder on that type of wire..I had difficulty getting my dimmer switch wires to solder--they were all black inside,moisture evidently penatrated the insulation somewhere..

...I hate corrosion and rust!..
