How do you attach cables on planes, the type that extends from the back of the canopy to the tail?

Are you referring to antenna wires?

If so, I use very thin wire. I have tried thread, monofilament and invisible thread and wire seems to work the best for me. It will not sag with changes in humidity. To attach a wire I drill a small hole with a #80 (0.0135") bit. I insert the wire in the hole, dab some CA onto it and slide it into the hole until the CA contacts the model surface. I hold it in place for a few seconds until the CA hardens. I then attach it to the tail. Lately I have started making attachment points out of 0.030" steel wire. I flatten the edges with a Dremel cutting wheel until the wire is no more than 0.010" thick. I then drill a hole in it with the #80 bit. I drill a hole in the vertical stabilizer that is about 0.032". I cut off the section of 0.030" wire and insert it into the hole and secure it with CA. This is usually done during the construction phase. The wire is inserted in the hole and wrapped around itself to secure it. I have taken to using small pieces of hypo tube as insulators. I slide the piece of tube over the wire and CA it in place. It sounds like a lot of work but it really isn't and the results are second to none in the realism department. It is manageable in 1/48 and a snap in 1/32.

Cut and paste this URL to see what I am talking about. Tripod is not cooperating at the minute as far as links are concerned:

http://members.tripod.com/scottsmodels/images/1cantennapost.jpg

This is the tail of the F4U-1c conversion I have in my gallery

:-) Excellant Corsair, stm
You wire method sounds like a good idea, and looks good too. For someone trying this for the first time, or not being satisfied with their current method and looking for an easy alternative, then I think that the simplest method is to use invisable mending thread. Using wire requires you to add tension, to keep it taught, I guess?
When using invisable mending thread, I use the smoke coloured one, anchor it first. Either drill a hole in the fuselage, if the wire exits from there, like on the Spitfire or Hurricane, or simpley dip the end in CA and hold it to the top of the antenna. If you are building a Spitfire (who isnt) or a Hurricane, when secure in the hole, pull it up and, with a dab of CA on the antenna fin, hold it in position while the CA hardens. Now in both cases pull the thread over the rudder anchor post and add a drop of CA, let it harden, then cut off the surplus. You can cut or file a notch in the antenna and rudder anchor post, if you like, this will make it even more secure, but it isnt absolutely necessary. While doing this you may have ended with a nice amount of tension in the thread, but, if not, try this ( take all the obvious precations) hold a pin in self locking tweezers, and heat it until it is red hot, then pass it under the thread. Because the invisable thread is Nylon it will shrink when heated, creating very nice tension. This is a very simple and straight forward method, which looks good :-)
Mal