OK, I have the antenna out of the car. I need to open it up and am trying this method, but I'm having a difficult time prying the snaps open. Does anyone have any ideas on how to pry the snaps open easier?

https://www.ifixit.com/Guide/Disassembling+Mercedes+W123+Antenna+Motor/20902

You likely have this figured out by now, but the answer is to open them one at a time with a flat blade screwdriver.

This takes a lot of time and effort since when you just about get one out another one will snap shut. So I always use two or three screwdrivers and a helper to first work one out a bit and then another and then another. You have to go in order, that is do one and then the next one to it.

And I have found it easiest to start at the bottom and then work your way up both sides at once. You do one side and your helper does the other. That is, of course, after you have released the bottom snaps to some degree.

I have a similar antenna question on a 300td. My motor works well. Is it possible to replace just the mast? I can't seem to find that part by itself.

Yes, the mast can be replaced. The mast is available (check Pelican parts or EBay) as either a serrated edge or smooth edge plastic cord that attaches to the metal mast. To remove the old mast pull up on the mast while having someone turn on the radio with the antenna switch in the fully up position. To replace the mast, turn the radio on, feed the plastic cord into the antenna and turn the radio off. The mast should feed into the antenna.

If all of the mast will not come out you will need to open the antenna and remove any remnants of the old plastic cord stuck in the antenna.