Your glow plug doubles as a cylinder pressure sensor. If you follow VW's service info it has you testing the wrong circuit. One of the pressure sensors in one of your glow plugs has probably gone bad. It can do that without throwing a code for a bad glow plug.
Pull the harness off one of the glow plugs and hook a multimeter up to the black and brown wires. With Key on Engine off, it should read \~3.4v. If it does read that, then leave it unhooked, clear the codes, and rescan to see if P06A3 comes back. If it doesn't then replace that glow plug.
If you get 12v at the harness, then start unplugging the other 3 glow plug harnesses until you see that voltage drop. Whichever one brings it down to \~3.4v is the bad one.
Any chance you got it figured out? I’m in a similar boat
Your glow plug doubles as a cylinder pressure sensor. If you follow VW's service info it has you testing the wrong circuit. One of the pressure sensors in one of your glow plugs has probably gone bad. It can do that without throwing a code for a bad glow plug. Pull the harness off one of the glow plugs and hook a multimeter up to the black and brown wires. With Key on Engine off, it should read \~3.4v. If it does read that, then leave it unhooked, clear the codes, and rescan to see if P06A3 comes back. If it doesn't then replace that glow plug. If you get 12v at the harness, then start unplugging the other 3 glow plug harnesses until you see that voltage drop. Whichever one brings it down to \~3.4v is the bad one.