Sounds like you need to pull back / remove the trunk lining that is covering the antenna, and see what's loose. I think the hollow shaft from the antenna motor/whip housing is aluminum (fragile) so a gentle approach is the order of the day. If you have a small strap wrench, maybe you could counter-hold the shaft with that so you can loosen the nut?

Could be that someone "repaired" the antenna before, now you've got to deal with their mess.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC