Just finished this mod on my defender 300 tdi. Thought I would share it. The flexi conduit and fittings (20mm made by tower) were purchased from screwfix for under £12, and is enough to do two defenders.
The facia covering the left side screen blower pipe is wide enough to cover the two additional 20 mm conduit pipes on the left side. I did have to remove the blower funnel, as the scuttle needed to be elongated to allow clearance for the two pipes, then re fitted . The pipes terminate in to rubber grommets in the heater manifold. I marked round the facia and cut around the glove box plastic base tray to suit. Works a treat.