I have the seat heaters too on my wagon - a W124 300TD turbo of 1990.
There where you say I see a small black box about 1"x1 & 1/2" and 1 & 1/2"
high. Inthere are 2 fuses of 25 A in serie. The lid simply lifts off.
Someone told me it was for a mobil carphone which I toke out.
Well, I think I will try out if it is for the seat heating.
Anyway, it is definitely an MB arrangment but maybe not factory fitted.
Tell me when you find out.