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I do not enjoy electrical work but this task must be faced when rehabilitating an old beater. My current project ('55 BSA A10 Gold Flash) will require installation of a new harness and renovation of existing switches (dimmer with soldered leads). My question is; what is the best tool for soldering and unsoldering on a 6 volt system (relatively heavy wires) ? Gun or small iron ? What wattage ? Any particular brands to pick or avoid. Any guidance is appreciated.

For most automotive, motorcycle, and aircraft jobs I use a good (antique) solder gun. Little to nothing on these applications is fine detail work warranting a bench top soldering iron. If we were discussing component level circuit board repair, my answer would be different.

I prefer crimping rather than soldering, but when I must solder, I like a low wattage 20-30 watt Weller brand with Weller pretinned tip. They transfer heat well and only put solder where you want it, and stay clean with a wet sponge wipe. Solder doesn't make for a strong wire strength because it causes the multistrand wire to break after a few bendings, that is why they recommend a crimp. sometimes I solder the bullets with a very fine solder just a tiny bit through the hole on the end, but it doesn't reach the wire side, If there is a strain relief crimp, I put a little solder past the crimp body near the business end of the connector to assure it wont pull out. imho

I had a 30 watt iron that died. tI did not supply enough heat to solder heavy wire into a brass fitting like a switch. I guess a 125 watt gun may be the way to go. Thanks for the advise.

I had a 30 watt iron that died. tI did not supply enough heat to solder heavy wire into a brass fitting like a switch. I guess a 125 watt gun may be the way to go. Thanks for the advise.

125 watt is too high. Hard to control for small jobs. As previously stated 20 to 30 watt is fine.

When I use a soldering gun on motorcycle wiring the one I use has a dual range of 40/100 Watts. Although 40 Watts if fine for most things, the 100 Watt setting often is quite useful.

I find that my Weller WLC 100 soldering station is up to most tasks with the chisel tip. Variable temperature deals with varying jobs. The only time I use it at full power is when working outdoors in cool weather. Other tips are available for more delicate work.

And add 2 pieces of heat shrink to the joint to increase the mechanical strenght of the joint. This appies to both solder & crimp joints.
I fix mowers and it is very common to have a wire broken at the terminal creting a random open circuit.
The pices should be of different lengths and the heat shrink ideally would be the glue type .
Three would be even better but tends to get bulky.

....and a small pair of vice grips attached gently to the wire acts as a heat soak and a handle. It helps prevent heat travelling up the wire, softening the insulation and shrinking the heat shrink before you get the chance to slide it into place.