they actually make pre-soldered terminals...just have to put the wire in, and it melts on... They are used for bigger truck applications..
Besides that.... if the terminals are closed at the end... put a bunch of solder in the terminal...melt it all with a sharp flame mini torch...keep feeding solder in it until its about half full, then quickly stick the wire in there and it should be fully soldered to the top... once the wire is in...careful if you try to add more solder because you will melt the wire coating.