Description of glowplug operation from Defender 300tdi manual: Glowplugs are fitted to the TDi engine to assist starting. A constant battery feed
is supplied by fusible link 3 (C907-3) to the glow plug timer (C189-5) on a
brown wire. When the ignition switch is turned to position "II", a second feed is
supplied to the glow plug timer (C189-3). This triggers a feed (C189-6) to the
glow plugs.
With the ignition switch in position "II", the glow plug warning lamp (C230-4) is
provided a feed via fuse 17 (C580-33) of the passenger compartment fuse box
on a white wire. The warning lamp (C233-10) is connected to the glow plug
timer (C189-2) by a yellow/black wire.
When the glow plug timer provides a feed for the glow plugs (C189-6), it
simultaneously provides an earth path (C189-4) for the warning lamp. After a
set period, feed to the glow plugs, and the earth path for the warning lamp are
cut; extinguishing the warning lamp.
Hope that helps.