My glow plug light on the dash blinks once my.

Under the vehicle at the back (in front of the rear axle passenger side) there is a plastic container. That is the Water Separator.

Loosen the round knob and catch the fluid that comes out of the spout. A glass jar is great for this as you can then see if water
is in the mix. Water will settle at the bottom if any is present.

There's a sensor at the top of the container that (when it detects water in the fuel) causes the light to blink. There's a relay that
is part of the system, so it could be a faulty relay as well.

Thomas