You may want to check out this thread I posted awhile back. Feel free to ask more questions, I have done a lot of work with Bachmann locos and have used their tenders behind lots of other brands.
http://cs.trains.com/mrr/f/88/t/181314.aspx?sort=ASC&pi314=1
None of the wires are bare, the drawbars does not provide an electrical path. Yes, two are the motor, two are pickups, two are the light.
Not all tenders are wired with the same pin locations in the plugs, this is done because some locos have LED headlights, other 12 volt headilghts - swapping tenders would burn out the headlights in these cases.
But swaps can easily be done and the pins can be relocated in the plugs.
I have done a long list of Bachmann tender swaps, too much to list without a specific question.
Removing the capacitors as others suggest will improve slow speed performance quite a bit.
Remember, all of this applies to most all Spectrum locos made in the last 15 or more years as well as newer regular line locos like the Berkshire - not just what is currently available.
As noted by others, Bachmann uses color coded wires, but not NMRA colors. It is easy enough to ring out or physically trace out the wires on the tender end.
What model do you have and what would you like to do? I can likely describe the easiest way if I know what you need to do.
Sheldon