Hey Ted
Not to say that anyone who has given advice is wrong here but 25W iron is fine, thats what i use, mines an Antex 25W. As stated its imperitive that the tip is clean. It should be shiny - if its not use a flat file on the tip. The tip needs to have a flat ideally - not a pointed one ! I constantly use a bit of wet kitchen towel to wipe the burned flux away after each joint. You need only tin the ends of the wires and also tin the speedo where you are trying to solder to. It helps if you have decent rosin core solder - the Schumacher stuff is good as is the MuchMore stuff too.
I tend to mount the speedo onto my workbench with some servo tape ( Losi stuff is good ), clean the tip, apply a small amount of fresh solder to the tip just prior to use to help with heat transfer, then apply the heat to the speedo pcb whilst pushing the wire through from underneath. If you hold the heat onto the wire itself for too long then it will draw the solder down it making the wire rigid and youll have a crap dry joint too !
After a few practice runs you`ll be able to acheive a factory shiny joint without too much trouble. If you remove the heat a fraction before you remove the solder it helps to get that smooth shiny finish on the joints as below.
Hope this helps
Rich