This junction in trunk was damaged during attempt at repairing broken antenna (not by me). It's in a tight spot behind back seat. Is there any way to mobilize cable? I would appreciate any suggestions.

There are multiple ways to fix this, including replacing untenna with aftermarket amplified antenna and hiding it out of sight.
If this would happaned to my car I would replace connector. On head unit side it is FAKRA connector. I am willing bet $50 it is same on other end.
FAKRA connectors:.
ou will need crimp tool as well

Unfortunately there is only about 6 - 8 inches available on proximal (radio) end to work with. There is simply not enough room to make repair. Is there a way to access more of that end from inside car...i.e., back seat or the shelf behind seat under rear window?

Definitely, will have to remove seat and back.behind seat back there is a plastic pass through for wiring, one on each side.. you can pull antenna cable inside passenger compartment do repair and route cable back in. Or, you may elect to have a patch cable. That may diminish antenna reception.
anyway, you will need to pull seat and back support out. Read how to unlock latches for rear seat, if you have not done that before.

Hi Eggdoc41,

Looking at your signature, is the year of the car you are asking about actually a 1986 XJ6?
You have posted in the X350 forum and drawn a response based on that model.

The plug style you have is the conventional RF DIN plug which was the industry standard RF connector used in the auto industry for decades.
There are solderless plug options for cable repairs, or you could probably pick up a pre-made extension cable to replace outright or customize to fit.
But for the physical access and cable pathway questions you have, you may be better served posting your question to the XJ Series 1, 2, and 3 forum members.

I stand corrected! I should stop replying from smart phone!

Nah....keep up the good work pal.
For all we know he might also have a X350, but his photo does seem to lean towards an earlier model.