Please help me ID this loose hose!

[Maroon 300D]

So I put in new glow plugs today and that seemed to go pretty well, but when I was done I noticed I had a loose hose. I'm really not sure what this is for exactly and it's unclear to me where it goes. I'm thinking it has something to do with fuel or the vaccum system as it is a clear hose. It seems to originate from somewhere near the firewall on the driver's side, but it was getting dark and I couldn't completely track down the origin.

I also removed the instrument cluster a couple of days ago so I wonder if I could have pulled this hose loose while pulling the still-attached instruments toward me...on the other hand, it seems very plausible that I might have knocked it with my hand or a wrench while removing/installing the glow plugs.

I did find one possible place where it looked like the hose could go, but I'm not entirely sure any hose connects there. It's a thin-nozzle type thing a little ways beneath a red plastic cap that is next to the engine, on the passenger side. It is toward the firewall. I don't know what purpose this red cap serves.

I know written descriptions of these sorts of things are difficult to grasp, but I am so far unable to get my pics to a file size where I can upload them here. We've tried zipping and trying to make them this or that file type but we can't get anywhere near 65 kb. Any tips in this regard?

I can try taking some really low res pics tomorrow, but I guess I'll post this now in the hope that one of you out there has had this exact same issue before.

Thanks for your help if you can give me any!