Jim,
I have checked my schematics and now I remember why I did not do the resistor .
On the E320 the ETC sens.[B11/3] feeds the HFM-SFI mod-N3/4. That runs the aux fans and other systems.
The other ETC , [B/13, single wire to gnd. ], is gage sens. only. It is on the neg. side of the gage and serves no other purpose that I can see.
Different set-up than yours in the sense that there is no CTS. So that is where you are catching the trigger for the fans without worry about N3/4 management.
I can't do that . My data list no CTS, only pressure at drier for low fan with a/c.
I rarely use the cabin sw. I installed , but I have done this to most of my previous models and there are times
it has been handy.
Would be nice to go the auto route though.
I'm Thinkin' !!
Arthur