Time to get the schematic and find where the +12v feed is grounding.

The fuse block has a lug connection underneath. However, since the circuit feeds other functions, it's unlikely that you can disconnect just the antenna feed at the block. It may be a common +12v feed that branches off inside the car to feed the other functions. If you are lucky, you've got a dedicated feed for the antenna only....

If so....

It's not too hard to replace the wire, the harness runs along the threshold plates, you can pull them, and pull back on the carpeting to reveal the harness. Removal of the back seat will get you to the grommet that goes into the trunk above the wheelhousing. Only difficult part is removal of the fuseblock and feeding the new wire into the passenger compartment. Use the old wire as a fiche to pull the new one through the firewall.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles