I hate those little plugs that Bachmann uses because they cannot be replaced by the end user when a wire pulls out. I have catalogs from several large electronics suppliers and none of them have those plugs available.
You'll have to contact Bachmann and send it back to them for repair or replacement. Those tiny plugs are not available even from Bachmann as a repair part.
Two members of our local HO scale club have done the same thing over the years, and had to send the locos back to Bachmann.
Here's Bachmann's contact information. It might be best to send them an email explaining your problem because you may need to have an RMA number.
http://www.bachmanntrains.com/home-usa/contactus_2010.php
Wires pulling out of those plugs are usually not because the wire is too stiff, as you believe, but because you CANNOT remove the plug by just pulling on the wires.
The best way to remove the plug that I have found is to carefully pry it out of the socket with a very small flat bladed screwdriver, prying first on one side and then the other and carefully working it loose.
I recently bought an almost-new SP Daylight loco off EBay. It ran well enough to begin with (aside from its very low tractive effort). But now one of the wires from the tender has broken off the tiny plug that goes into the loco (wires too stiff, IMO). How can I fix this problem? I haven't seen tiny connectors like these, nor do I know where to get another if this one can't be disassembled. Any help would be much appreciated.