Thanks Y'all -
I decided to dig into the 124 wiring diagrams while sitting here eating my lousy cafeteria lunch. Going to compare the fan wiring for my old 124 (1987 wagon) to my new 124 (1993 300E 2.8).
On the old car, the pre-resistor is fed by a 2.5mm wire from the low speed relay. The (single) fan is in turn fed by a 2.5mm wire from the pre-resistor. Nice and neat, it works fine after, uh, 17 years.
On the new car, the two fans are fed by two 2.5mm wires from the pre-resistor. The pre-resistor is in turn fed from the low speed relay by a *single* 1.5mm wire. Electrical design by Homer Simpson.
I'm gonna follow Donnie's path and run a new, heavier guage wire from the relay to the resistor. Should be an easy enough task, and fix it once and for all.