Where can I purchase an led flasher relay
Very nice website, and thanks for posting that link, nib. Those guys REALLY seem to know their stuff … 
Just my 2cents on LED bulbs with the XJS. I remember when I bought Supercat (black/black '95 coupe, R.I.P.) that I played around with the idea of switching all her lights to LED. I admit replacing the interior cabin lights with LEDs was pretty cool-looking, in part due to the black interior, which contrasted nicely with the white LEDs. However, when I got Superblue, the LED interior lighting just looked “distracting”/mismatched (IMHO) with her doeskin interior color. I ended up switching her back to her original bulbs. I did the same thing with her LED boot lights for pretty much the same reason, as well as the fact they did not seem as bright as the OEM incad. bulbs. I like my trunk to be well lit, in case I need to search for something at night in there, esp. if broken down at the time. 
As to exterior LED bulbs, I didn’t actually try those, but noted that, like the website suggests, you have to use bulbs that are the same color as the lenses in front of them. I guess that would be fine for all the exterior lights on the XJS BUT the facelift taillights. Problem there is our wraparound taillights have a smoked-color exterior lens which is in front of the actual colored lenses (i.e. red and orange). I’m not sure how the ultimate light from the colored LED bulbs would be affected by having to pass through the smoked lens. Possibly not at all. ?
Before switching those bulbs for LED ones, I would want someone who has already done so to report on that potential issue (or even better yet, show before/after replacement pics/video).
There is also the issue of hyperflashing with LED bulbs and false “bulb out” warning lights, but it looks like that vendor does have “remedies” available for both of those unwanted situations. Still, I would prefer to hear from someone who has actually installed exterior bulbs on their XJS in regard to same, before I go switching the bulbs.
Finally, as to the instrument panel LED bulbs, does anyone know if the ones they show in their chart are the ones that actually just illuminate the instrument panel, or are the bulbs for the warning lights, turn signal, high beam, etc. indicators in the indicator light strip? If the latter, knowing how hard it is to see those bulbs in daylight, are the LEDs any better (i.e. brighter)? It looks like in their comparison videos of exterior LED bulbs, the incad. bulbs are actually a bit brighter than the LEDs. 
To make the warning lights “see” that there are bulbs there, get ones that have the “canbus” built in. It is a small resistor that matches the original specification of the stock bulb. You can also add a small resistor across the terminals.
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I have a 1986 xjs that I removed all ( and I do mean all ) the wiring out of. So I no longer have the bulb warning lights in the dash. I went with their recommended led color for amber and red. Of course being a pre face lift my tai lights are not smoked. I installed their resistors and all works well. One issue I did run into was the light bar in the rear window. installed new led mini lights and applied brake fuse blew. upon further inspection the contacts on the bulbs ran from one side to the other and the jag socket connectors are lengthwise ( hard to explain but easy to fix). Cut the wire on the bulb so only one side made contact. Basically it was a short. Once modified all works well.