Having worked in an auto repair shop or three I wouldnt run an antenna wire through any conduit considering what most conduits in a garage carry. Your shop probably has cinder block walls, if the boss is cool with you doing this you can drill a 3/8 inch hole through the block and use a thin but stiff wire to pull the plug end of your antenna through the hole leaving the antenna module outside on the wall.
The other thing is that the sirrius units do have a decent transmission foot print ( Ive tested mine to work up to 30 ft from its location ) and you could still use it even if its not close to where your at if you can say run the antenna out a door.