You've got two problems here: making the drive accessible to the service and granting that service permission to use it. I'll assume that you're running Win2K3 terminal services since you have drive redirection. You'll need to get that mapped drive a drive letter. The simplest way to do this is to share the drive from the client PC, grant that service's account permission to read it, and use net use to connect to it.
This was first published in August 2005