It's an easy task, although my friend had real issues on his 156. The tip broke on three of them and was left stuck in the engine block, and now he has only one working glow plug. Mine was pretty easy, just unscrew it, pull it out, put the new one in. I never used a torque wrench, but it can't hurt. As far as where to buy them, i can't help u there, since i live in Serbia. i paid about 15 euros for mine, but i think they are not the greatest, made by champion but probably made god knows where.
P.S: Make sure you are not doing this on a hot engine! And what i was told by a mechanic, if only one is bad, replace just that one, if more then one, replace them all.
Last edited by Nedster; 01-09-10 at 17:14.