Add flux. I use one of these: https://www.amazon.com/MG-Chemicals-Rosin-Flux-Pen/dp/B077TZ8GRM

Do you have a temperature controlled iron?

You could cut that zip tie and pull the wires out of the plastic wire clip to give you more length. Then at least you are not fighting that. The wires look to be oxidized so with more length you could cut that off and expose fresh wires. Can also clean up the switch terminals (fibreglass brush works great, but so would a scotch brite pad in this case). Flux as mentioned above really helps with cleaning the joint, but if you dont have any on hand try to get it as clean as possible without. The switch terminals are large which act as a heat sinks. So if your not already, add some solder to the tip of the iron before heating the joint.

It could be the type of solder you are using...

Do you have it on backwards ?

LTG : )

I found this to be my problem so when I cleaned up the terminals with a emery board ( nail file ) it seem to solder 100% better even with my cheap soldering iron.

Then you are good to go.

LTG : )

Use some sandpaper to rough up the terminals, then it will stick

And tin your soldering gun.

You could also add more wire to lengthen the originals. I have a bundle from a pin that I use to try and match the colors.

I wonder is somebody put that diode on there with a higher temperature solder, like silver solder or something ?

Did you put the diode on ? If so that went good. Using the same solder ? If so maybe your soldering iron is failing to heat up ?

LTG : )

Weird. I've always found them to get hot quick and stay hot and melt off solder that was already there.

I think I'd remove all the solder. And start over with new solder.

LTG : )

Clean the contacts, sand them, them clean them with 90% iso alcohol and dry and then tin them. Could have residual oil that will inhibit silder from sticking. If your wire can be tinned, you are good.

Most switches are mass produced in China. Those terminals suck and need cleaned like mentioned above.

Are there not holes in the terminals you can thread the wire into and secure mechanically? Good solder joints flow the solder over a solid mechanical/electrical connection to make them semi-permanent.

Stop buying cheap $5 irons. Likely whatever thin plating is on the tip is gone and no amount of tinning it going to save it. If the solder balls up on the tip, even without soldering to something, the tip is shot.

Others have already pointed out you can pull some wires out of the bundle to get more slack, or to add some jumpers to it (use heat shrink on the joint, and use the lineman's joint, not a pigtail. Whatever you do don't use electrical tape around the joint, it won't stay.)

+1. You buy crap, you get crap performance.

Remove all the existing solder. Get the wires to where you can reach the switch. Mount the switch BEFORE you solder anything. Cut wire down to get to clean wire if you have to, and make a mechanical joint on the terminal. You shouldn't need the solder to hold it in place, and there's plenty of room in those holes to fit the diode leg and the wire. I've done it a bunch of times. Add flux to the terminal and the stuff immediately touching it. Solder it.

If your switch doesn't have holes in the terminals...well....drill them some, or something. I dunno. I've never had that issue. :p

Use the holes for the wires. Add the diode after and just wrap the leads around since they stay in place being solid vs. the wire being stranded.