We had a 06 plate VW Passatt (same platform?) 1.9TDI PD engine that would throw MIL light on glow plugs occasionally. Never did fix it. We had new glowplugs fitted too which seemed to help for a while.
I had limted success changing a relay under the fuse board, under the bonnet - 2006 VW Passat B6 Estate 1.9 Tdi Bosch Glow Plug Relay 0281003038 - Certaily helped on our car - but changed it a few times over the years when the fault came back.
Felt was a loom problem and the relay was a placebo - But, I could clear the fault every time (weekly) and after changing the relay it was fine for months.
Ran fine, bit smokey. We got shot at 155K in 2018. She's still going strong looking at the MOT history - 221K in July 23 🙂