The insulation may break down, but the wires would still be there. As long as the connections are good and the wires haven't been mangled the copper should be fine. Age shouldn't affect the ability of the metal in the wire to carry current; broken strands will have a much greater affect on the current carrying capacity. Broken insulation may lead to corrosion of the wires which would also lead to reduced capacity.

That said, one way to check if you have a problem in this area is to measure the voltage to ground at the different points while the glow plug relay is energized. Measure at the battery post, battery terminal, fender terminal and glow plug post where the large wire terminates. Ideally, these should all be at the same voltage. Voltage drops will tell you if connections are bad or wires are bad or battery is bad.

Another thing you can do is run a wire directly from the battery to the glow plug terminal. You could rig it up temporarily to see if that solves your problem.

And check the fuse again. The nuts may have loosened up. Ya never know.

__________________
mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)