Just press fire.  You need to be looking backwards and aiming requires reversing the control inputs that you make and takes time to get your head around.

There is a freeware program that helps you with this but I can never get it to work on my system. 

Will

Yo Can_Sapper,

  Try the "2" key on your keyboard to fire the  rearward facing weapons.  If this doesn't work try the "space bar" on the keyboard.
  The "spacebar" should fire all weapons (front, rear and side mounted) at once.
  The "2" key is usually associated to firing cannons only, but some third party designers have accessed this key to fire rearward facing weapons.
  If all else fails read the "read me" that came along with the third party aircraft.  Most designers are good about listing whats special on their aircraft and how to access it.  Some third party designers use the "2" key to fire rearward facing weapons, some allow all weapons to be fired using the "1" key (or joystick trigger) and some did not build this option into their aircraft.

Yo Can_Sapper,
  Did you get your tail guns to work?

Thanks for the replies all.  Those key strokes you told me about do work for some of the a/c.  I guess the others weren't programmed.  I've also downloaded tailgunner and will have to see how it works.  Hooray for CFS1 it is still tops as far as I'm concerned!

cheers.