Misery loves company.
I am battling a very similar problem. Have had it since I've owned the car (about 6 months), so I don't know how long it has been doing this. Like you, winter is coming and I want to resolve this issue before frosty comes. A simple 2-wire system like this? An easy fix.This isn't rocket science.
Or so I thought.
The seemingly simple problem I am battling is that the glow plugs don't stay on long enough. The glow plug light comes on and goes off in about 1 second (or less). Doesn't matter if it is hot or cold. When it IS cold, it requires 2 - 3 cranking sessions to get 'er to fire. I know this isn't right.
When I pull the plug to the temp sensor, the glow plug light stays on for 10-12 seconds. I am only asking for maybe 3-4 seconds when it is cold... Is that too much to ask!?!?
I've been through the 101 and V2 and saw a ton of great advice, and followed may of the suggestions, but still haven't hit the nail on the head.
I've had the P0380 coming on constantly over the last week or so - had never really done that before. I was resetting CEL each day, sometimes multiple times per day,(sometimes ignoring it) but it was always the same code. I finally received my new harness. After installing the harness, I found that the previous owner/mechanic had done the 'join the wires together' modification. I decided to put the harness back to OEM configuration with each wire soldered and shrink wrapped and hoped for the best. So far I don't have a CEL any more (3 days and counting), but I don't think the problem is resolved either. The GPL stays on for the same amount of time, and the starting really isn't any (noticeably) better.
I can still pull the temp sender and get a good 10+ second glow out of it, so I know the system integrity is fine - it seems like it is just not getting the 'signal' to glow longer than a flash. (Or is being told to stop?)
Yes, I tried the VAG-COM glow time extension mod, but it didn't yield any results either. Still just a short 1 second 'blip' of GPL.
I have access to a shop, a VAG-COM, the car is parked in a garage (been parking it outside to try and keep it 'cool'), has a pan heater, and I have another vehicle I can drive if something goes sideways on a repair. It's not that I am in a panic to fix this issue, it's more like I am on a MISSION to fix this issue.