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[TheBoy]

In HVAC, got a B11ED-87 Electric Heater Control Module

Is that an axe PTC heater to help warm cabin until engine warms?  Looks too much of a bugger to change if that has gone faulty.

Schems show it's powered via a 150A fuse, but I can't find a 150A in the usual fuseboxes?  Schems are Part 2 under HVAC in Topix's wiring diagrams.  Any idea where this fuse could be, as worth checking before going further?

[YZ250]

Stumbled across this:

The source was Jaginfo.org

‘The megafuse is in the battery junction box in the boot on later cars. It’s 150 amps.’

No idea if it applies to yours or not. 

Although that may be for the XE, XF, and not the XJ. 

[YZ250]

It’s a bolt in fuse, apparently in the trunk side BJB, not a normal blade type.    That info was in reference to the XJ X351, so you probably know all of this. 

Schematics shows Battery, to Battery Junction Box (150A Fuse) , PTC + going to Heater Electric Booster.

[TheBoy]

Yup, cheers YZ, thats what I'd found in the schematics, but it doesn't say where it is.  I'm making an assumption that a Battery Junction Box is near the battery, much like the fusebox on to of the battery's +ve terminal on an Omega.  XJ has nothing like that, and the access panels in the trim in the boot only give access to microblade type fuses.

[YZ250]

Yup, cheers YZ, thats what I'd found in the schematics, but it doesn't say where it is.  I'm making an assumption that a Battery Junction Box is near the battery, much like the fusebox on to of the battery's +ve terminal on an Omega.  XJ has nothing like that, and the access panels in the trim in the boot only give access to microblade type fuses.

Yes, I’d agree with your assumption that you should be able to just follow the battery +ve lead to the closed cover junction box close by. 
Hope you manage to find it. 

[TheBoy]

If I have to start taking the side trims out, that becomes a weekend job.  Same if it's behind the rear seat backs (where there is a shit load of technical stuff)

[Doctor Gollum]

If I have to start taking the side trims out, that becomes a weekend job.  Same if it's behind the rear seat backs (where there is a shit load of technical stuff)

Sounds like the Jaaag engineering team needs to spend time poking around a W221 to see how to make things accessible.

Merc put everything behind the rear bulkhead. 4 T25 screws, and 10 push pins and the boot is stripped. Hardest part of working in there is reaching from the rear bumper and getting back out of the boot after doing the work