O.K., now that I have the security code in hand, I tried turning the key to Pos. 1 and, if the system works like it does in my face-lift XJS, after powering it up, a message should appear on the radio dial inviting me to put in the code #s to re-activate it. However, I get nothing - it’s like there’s no power going to the system at all. Even when I push the blue (cartridge) “eject” button on the CD changer in the boot, nothing. 
I’m wondering, again, if it’s the unprogrammed key of mine that is to blame (and please, I know about Chris Farley already, Motordude 
). 
The only other thing that I would think that would cause that “out” condition is a blown fuse, possibly due to her snapped-off antenna mast (I suspect one of the people I lent it to forgot and left it extended/radio on while in an automatic carwash 
Been there, done that, 2x myself w. Jags ), but then I was thinking there are separate fuses (circuits) for the entertainment system and for the antenna motor up/down circuit. Also, when I snapped them off myself on the other two occasions, it did not blow a fuse anywhere.
When I had my off in the XJR the radio stopped working, the fuse was in the footwell in the rear behind the driver (RHD)
You did try pushing the vol knob didn’t you ?
Just saying
Yep, and actually it looks like the system uses THREE fuses. The one that Robin mentions is the one for the “radio/cassette head unit” … In the big boot fuse box there are also fuses for “sound system” (IIRC = the speakers ?) and one for the antenna motor. I’ll check all three. Harlem doesn’t have the Premium sound system, so there is no high-powered amp for the speakers, else there would be FOUR fuses to check. 
In follow up, I today pulled the 10-amp fuse in the left heel board for the “radio/cassette unit” (i.e. head unit) and indeed it was blown. After replacing it with a new one, though, I went to power the unit up and STILL nothing. 
I have not yet gone back to check to see if the new fuse has blown as well, but I’m suspecting now it probably has and I think I know why, if so (and why the fuse for the antenna motor itself is not blowing) …
I notice that with the XJ8s the setup for the antenna motor is that apparently it does not utilize relays, unlike earlier models of Jags, which have a pair “up” and “down” relays. I say this b/c there are no relays for the antenna motor contained in the relay info in the Driver’s Care guide. Instead, I assume that this function is now subsumed in the radio/cassette unit itself, possibly by way of a processor or control module inside. (?) As we know, when the radio/cassette unit is first turned on, it goes into “radio” mode and therefore extends the antenna. If the broken off antenna mast has resulted in a physical “jamming” of the motor system, then possibly that situation triggers the radio’s fuse to blow, cutting off power to the antenna motor and, importantly here, before the antenna motor fuse has a chance to overheat and blow. Does this all make sense? If that’s the case and is what’s going on, then I wonder if I pull the antenna motor fuse if that will keep the radio fuse from blowing (i.e. b/c current never even gets a chance to go to the antenna motor then). ?
Anyone?
Sounds plausible, pull the fuse and see.
btw, something else I noticed when I was playing around with the function buttons on the radio today (i.e. the ones all along the bottom row in the control panel). I discovered that some of them were now totally immovable when pressed, as though they were supposed to be fixed in place like that. After pressing HARD several times on some of them, I got them to finally start moving/working again. I think the only one I didn’t break loose was the “A/T” one near the right end of the row and the other one next to it. I SUSPECT what may have happened is that one of the persons who had possession of Harlem during that interim period may have spilled a sticky/sugary beverage into that area, where it leaked down (due to the downward slant in that area) behind the buttons and, over time, “glued” things together. If the radio was in fact inoperable/blowing fuses at that time, the driver wouldn’t even be aware of the situation with the buttons b/c they wouldn’t even be using the radio anyway. The buttons themselves were not sticky, but I do remember using an auto interior cleaner foam spray earlier, so it may have removed the junk from them at the time. Or possibly the person that spilled it - one of them had 3 hyperactive young rugrats, btw - just cleaned the face of the buttons and didn’t do more. ?
I’ll try carefully prying off the buttons from their stalks (if that is possible?) tomorrow and spraying electrical contact cleaner in that area very well, then pressing the stalks several times to free them up. btw, I don’t think that condition could be responsible for the radio/cassette unit fuse blowing (esp. after r/ring it) as I don’t see how one or more of those buttons “stuck pushed” (closed) could alone cause the fuse to blow. Is that correct?
I don’t think it’s the antenna. But just unplug it to rule it out.
It has a power input
And a separate signal from the headunit to command it.
Someone on “the other forum” claims that it’s possible that if something was spilled into the head unit that, despite its fuse blowing, something inside could have burnt out as well (and if so, I’ll have to r/r the radio unit). I thought the whole idea of fuses was that they are designed to blow first and INSTEAD of damage being done to the components in their circuit- sort of a “sacrificial” thing. So is it possible both could have happened? 
Its possible. Maybe 65% likely.
Sadly fuses mainly there to protect wiring chaffing against metal and causing a fire, rather than protect devices.
If the fuses has blown you can replace if with a 55w bulb for diagnostic purposes. Then disconnect the head unit and/or move the wiring about. When the light goes off you have found your fault location.
ICE wiring runs down the middle of the car under the arm rest. I assume the power does too?
Just found out on another radio-related thread that possibly there is an inline fuse with it … (pigtail, maybe? ) … That’s what the facelift XJS radios have, so if the X-308s do too that’s probably what (else) blew … I’ll have to pull the radio to get to it … Wouldn’t the electrical schematics show one, as well?
I doubt the x308 has any inline fuses.
Well, got the replacement radio in, took out the old one, went to connect up the new one before actually installing it in the dash, and no power going to it, either. Checked the 10-amp fuse in the left side heel board panel to make sure it did not blow again as with the original radio, and it hadn’t. I have written the vendor that sold it to me to make sure he tested it before sending it. Since he is very experienced with XJ8s/XK8s I also asked him about my theory about a programmed key being required for the radio/entertainment system to power up.