Are you using a flat tip or a point? If you're using the fine pointed tip it will never get hot enough. I'm using a Hako soldering station with the stock tip it comes with and I can do the wires on that speed control just fine. I've doone it a few times. With the external solder tabs, hold the iron underneath the wire like you were. You might just need to clean the tip or if you're not tinning the tip with solder first, do that.