glow plug circuit fault

hi guys. im currently having a problem with the glow plug circuit on my 2008 fiesta 1.4 tdci. it started a while back when the engine management light appeared. a scan indicated that number 1 glow plug was down. on trying to remove the plug i discovered that it would unscrew but i coudnt remove it from the head. ive since been told that its common for the tip of the plug to swell making removal difficult. as the car is in daily use i decided to strap a new glow plug to the egr pipe to attain an earth and re connect the feed wire to put the engine management light out. this was a temporary solution  and i was aware the that the car would only have 3 glow plugs functioning correctly. i went under the bonnet yesterday with the engine running and noticed that the the glow plug i strapped to the egr pipe was glowing constantly. on switching the engine off it immediately cooled down. i turned the igntion back on and the glow plug warmed up and cooled back down when the relay kicked out. on starting the engine, the plug warmed up again  didnt cool down until i switched the ignition off. the only fault on my scanner was that the fuel temperature sensor was reading minus 50c. this has been rectified and the fault code erased but the glow plug fault remains. any help would be appreciated . thanks.