Somewhere I missed this post...shame on me.
I'm assuming you have wood framed walls you are going through.
Just cut a hole in the interior wall in the area you will be going to the outside. This is to see if anything is in the way inside the wall. Keep the vent line on the inside wall as much as possible. Then where it goes to the exterior just install a vent with a flapper or back-draft hood so strong winds do not come back in. You will always loose some heat from these, however it should be OK.
Posting some pictures of the area will get better answers.
Good luck, and tell us how you make out.