The metal ends are part of the wire, they are NOT removed independently of the wires.
There is a 17mm wrench that was originally to be used to adjust valves on the inline 6 M110 motor. THAT is what MB recommends be used to remove the plug wires from the plugs. I believe it is tool # 110-589-01-01-00. Here is a place that seems to have it for sale for about $50-$60:
http://www.samstagsales.com/mercedes.htm
You may be able to get them off using a spark plug wire pullung pliers, but this 17mm wrench has a really nice bend to it that makes it lever off the ends really nice.
The open end part of the wrench goes onto the narrow part of the metal end and then the wrench is pried against the valve cover to wedge off the wire ends. I recommend also removing the coils from the valve covers to do this job.
You will want to remove the engine cover to do this job, I usually remove the air cleaner box also to get better access to the right bank.
You are actually overdue for this job by quite a bit, as the plugs also have a time expiration of 4 years on a 1998.

Gilly