I agree with the last answer. Have just had the same problem with my Townace diesel. If you have a multimeter with ohms resistance scale disconnect the glow plugs (there is one per cylinder, they look a bit like small spark plugs) and then measure the resistance from the plug connection with the other meter probe on the engine block or any earth. if the plug is good the meter will show a very low resistance, probably less than 1 ohm.
On my van all four were faulty (have done 196 000 km) I only replaced three as the back one is hard to reach, but it seems to work fine with three.
Brendon
September 2008