Had a bulb blow on Superblue earlier tonight, and when I tried to r/r it I found out I couldn’t get the black plastic retraining ring (the one with the “little metal fingers”) to rotate and lock fully and firmly back into place, but only stay fastened on one side to the assembly. 
I can’t get down low enough to see what’s wrong back there 
My hunch is that either something is amiss now with the retaining ring or, more likely, I have 2 or 3 of the 5 retaining tabs on the back of the headlight assembly somehow broken off. 
Is either of these a common malady with our “aged” headlight parts? I’m thinking if it’s the headlight assembly, given how expensive used ones are and the PITA to r/r them, I wonder if I can use the forerunner of the retaining ring - that black rubber “doughnut” with 4 “ears” - in place of it, to hold the bulb in place, assuming I can find a used one of those? 
The clip kinda depends on the lens. mine have a metal retainer clip that you squeeze together, then the bulb comes out. Granted my headlights are 4 individual and are HELLA flat lenses - not stock.
Look here for Part numbers
I’m assuming your XJ-S has one big aka euro style head lamp. Mine don’t - well one does but I haven’t taken it apart (yet). Anyway get the part number start searching the internet. SNGBarratt is a good resource, Terry’s Jag is another. for used parts try everydayxj.com Highly recommend everydayxj for used parts. Great guy!!!
At least one of my retaining clips have broken off. they get hot, and brittle.
let us know how you get along