Dear Diesel Demons:

Thanks for all the replies.

It's a blue hose, runs along the passenger side of the vehicle, from the receiver-drier and into the firewall. I did some more research yesterday and it seems to be the the one described as running from the receiver-drier to the expansion valve.

Is the receiver-drier that object located behind the passenger-side headlight bezel, with the sight-glass in it? Or is that some other component?

All best,

Diesel Fitter