That might work. I haven't looked hard at the diode idea but it might work for the blower. The aux coolant pump is wired through the PBU and I don't know exactly how it is powered. A DPDT relay might be unavoidable there.

Yes, the blower controller's power is switched off by the ignition switch. The controller will draw a small amount of power even with the yellow speed-control line floating, i.e., at zero voltage. It draws enough current to make a tiny spark when power is connected to the battery via a clip lead. Enough that you wouldn't want to leave it connected all of the time.

A diode in the main power lead to prevent back-feeding would have to be a healthy one and probably heat-sinked to boot since the blower can draw as much as 28 Amps at high speed. The voltage drop across the diode would also prevent the blower from reaching maximum speed in normal operation, probably not a big deal for most people.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970