I've had several 606's in the shop in the last few weeks with stubborn glow plugs. They were all in because of non-functioning glow plugs and when we went to replace them they wouldn't budge. We see this every year as soon as the weather turns cold. Soaking them doesn't help. All we can do is try and twist em' out. The almost always break and we end up drilling them out. We've never had to remove the cylinder head. Once we get them out we remove the pre-chambers. This makes it simple to get the debris out of the cylinders. After the job is done, don't be surprised if the engine knocks like crazy for a short time after it's started up. Lots of junk gets knocked loose when doing this job. It takes a short drive to clear everything out. Good luck