I do not know the exact mod., but I believe they went a heavier feed wire between the relay and the resistor. If you check the schematic I sent , you will see that wire was/is a smaller gauge than the high speed circuit uses.
I also think they fused that wire . The reason I think so is b/c the E320 version used two relays and they had a fuse for each speed , a 15 for the low and a 30 for the high. Yours just uses a common 30 A for both, so I think that is why the wires burnt/relays failed on the low speed circuit.
Just my hypothisis....