Right-o, got the holder on the key with the above advice. Thanks, guys. The tab does indeed have to go in at an angle with moderate pressure.
And yes, the new key holder is thin plastic, and it's obviously less durable than the original hard rubber. MBZ ain't upgrading every replacement component; some of them are being de-contented.
I went and bought another one of these for my primary 500E key, which was just starting to tear across as well. The dealer didn't have a part number for the holder on a 500E, but I just bought another 380SL holder. Seemed as though it would fit okay.
Cut off the old hard rubber holder from the 500E key, as I did before. But when I went to insert it into the new plastic holder, I found that the base tang is too big for the 500E key to fit the 380SL holder.
Drat! I've just ruined this key until I get a holder that fits. Back to the parts department to get the right holder.
Uh, er, turns out that there -is- no holder that fits. MBZ doesn't offer replacement key holders on the W124. If your key holder tears, they generously offer you the option of having a new key cut.
I'm displeased, both with them and with myself. Any suggestions?