Mark : While I haven't received my February MM yet, I assume you are referring to the rubber cap that fits over the top of the Dimmer Switch.
Jim - If I assumed correctly, I believe that you inadvertently pulled the wrong part number out of the 68 Electrical Assembly Manual. The Part # you listed is for a "Retainer" (# 37 in the attached illustration). The Part # for the Rubber Cap (# 39 in the illustration) is C5ZZ-13533-A.
Interestingly, this part shows up in the 1967 MPC as well as in the 1960-68 MPC, and in both, in the "regular" parts section, not in the Accessories Section. In addition, it is shown as being for 1965-68 Mustang Model 76 (Convertibles) only. To me, this means that it was "Standard" on the convertibles (while of course it could be used on any model). Another item of interest; the cap does not show up in the 1967 Electrical Assembly Manual, but as mentioned before, it does show in the 1968 Electrical Assembly Manual. Was this made "Standard" in 1968 for all Body styles ?
I have attached a rather poor picture showing the cap in place on my 67 Convertible; if you zoom in, you can see it better. I have also attached a couple of pictures of a supposedly NOS one that shows it in better detail (I don't have access to my NOS one, that I purchased from my Ford Dealer years ago, right now). These are usually available, both NOS and Repo, on Ebay.
Hope this helps.
Bob