Just so you know the reason mapping a drive doesn't work is because maps are per login/session not for the machine. If you have two users on the same machine they can have completely different mappings.
When you map the drive, you are doing it for your username and that session. IIS is running in the background as a different login/session. To be able to use the map in IIS you would have to map the drive in that session. Also just because of the nature of web requests, if you say mapped a drive through an action in a web page, no other web page could see it, and as soon as that web page exited the map would be gone.
This logic applies to any Windows service, since they all running their own separate environment.