I have a Mondeo 2002 diesel on which the glowplug warning light started flashing as I was driving home. No other warning light appeared and the car seemed to run normally. I was only on small country roads so I can't say if the performance may have been affected at higher speeds. Also, I can't say if the flashing occurred immediately after starting the car (it was still warm after about 30 mins in a shop car park) or some minutes later as it was a sunny day and I might not have noticed it immediately.
I checked the supply to the glow plugs. The 12V appears when switching on the ignition. After the usual amount of time that the glow plug light would go off, it flashes instead and the 12V supply cuts off after a further 10 seconds or so.
I tried starting the car a couple of times and it did fire but wouldn't continue to run. since the car has always started immediately I assumed that the plugs must be working normally, and that if one plug had failed the engine would start without too much difficulty. It seems unlikely to me, given the previous ease of starting, that it's a glow plug problem but they haven't been replaced in the four years I've had the car and I have no idea how long they have been in there. If I knew what current to expect on each plug I could check that.
I would appreciate advice on what steps are best to pin down the problem.