I took off the module from my 96 xj6 and found it had a plastic cap on the end. .
Was it supposed to be there or taken off?

 

Tom: you have posted this to the wrong forum; you want the X300 forum which looks after the cars of your vintage.
For readers: there was an ABS system for the Series III cars - but only in the 1991/92 V12 model: you can see it in the photo below - at the top of the photo where you expect to see the battery (LHD):