EZ wiring tail light circuitm

I did some testing today and it seems like the best spot for a ground is at the socket itself. What is the best way to go about soldering a piece of wire there? can I use my soldering gun, or should I pickup a butane torch?
Thats also where I soldered my wire to. I used a soldering gun, took a bit to get it to heat up. I will mention Im not great at soldering so you may have better luck. However mine has not come loose so I must of cleaned it properly.