relays for electric fan in truck

If there's any question about whether the ECU might be damaged you can add a transistor driver to buffer it. The ECU output would then only need to supply a very small current to control the relay.

The other advantage with open-collector outputs - they can all be wired in parallel. The manual on switch is effectively OC (Open Collector) (ie, GND, else open). If ECUs are GND switching, then they are usually OC (like, I mean, if you are grounding, why the bother providing any power/voltage when it is NOT grounded? Make it like a simple relay - its output is connected else not). Probably too the AC trigger if it is a single-wire switch - ie, a single-wire thermal switch... like a single wire oil-pressure switch, it can only switch to chassis/block/frame/body = GND.    By connecting all 3 together to the relay #85, you effectively have a logic-OR situation. IE - switch OR ECU or AC are connected to GND which means "fan on". Logic OR means and-or(like English - at least now officially in Australia)- ie, any of the switch or ECU or AC can be GND, but can also be switch and ECU etc or all (three) inputs. But won't you blow them up by paralleling?       No - that's the magic of Open Collector. Like points or ignitors for an ignition coil - you can't hurt a GND connection with another GND connection. (The IgCoil may not fire until all GNDs are "opened", but you won't wreck the points/ignitor etc if they are all closed to GND, or not.) Think of 3 GND switches to turn on a "hot" bulb (ie, other bulb end connected to +12VDC, 120VAC etc). So what if switch(es) 1, 2 or 3 are on in parallel? Different maybe if connecting 3 hot switches to a grounded bulb. We might then connect 1 to 2 to 3 hence interconnect power sources (+12V & +13.8V etc) or different 110VAC phases, or differently powered loads together.... Apols for the ramble... I should find that neat logic-equivalence diagram I saw somewhere..... Is that reasonably clear? An OC is like ignition points etc - it is either connected/shorted to GND, else not (and NOT means "floating" - a hi-impedance to any voltage etc).    OC may be a switch to GND (else open - like ignition points), or a transistor - specifically an NPN transistor being either "ON" to GND else off & floating. Its Emitter is connected to GND and its Collector is its output. When the transistor is on, its output (Collector) is connected to its Emitter = GND.       When off, the Collector is floating. It is not connected to GND/Emitter - that is now an Open circuit - hence the name "Open Collector" - it is open unless connected to the Emitter which is GND. Alas though OC is a brilliant system, I suspect my explanation is not. (That's Haemo's fault - I've hit the wine again!)

Bingo! End of story... But to confirm... (Oh no - here we go again. Aka BOHICA) If Open-Collector output.... When "off", the output merely floats. In practice it will tolerate whatever the "Collector-Emitter breakdown voltage" is, but this is usually above 30V so that even if our 12V systems hit 16V or 20V - no problem! And when on, it shorts (connects) to GND. It will tolerate whatever the output can "sink" to GND. (Source current if supplying power (voltage/current) to a load.) Although computer chips (CPU etc) might only sink or source 1mA or 20mA, an ECU should (or will!?!) have a buffer to source/sink more. Somewhere should state what they are capable of. But ECU outputs intended to drive relays should expect to drive (IMO) at least 200 Ohm relays (coils/solenoids) - though probably down to (say) 60-Ohms for automotive relays. (That's ~70mA (200R) or 250mA for 60R; where R=Ohms.) Great to see that following slowly reaps its rewards. Though I'm sorry if it's poor comms that makes it slow.... But slow is relative.... It's taken me years - how long's it taken you? (LOL - it's not a race! But I do know a lot of very fast idiots! Let's just say they burn out. LOL - get it, burn-out - unprotected over-current and no tomorrow... FIGJAM!) Yes dear, I'm coming to bed.... I'll just empty my bag....

Bedtime?  It is the middle of the day here!

I've been tinkering in 12V stuff for a while now.  I did catch a car on fire once so I am very careful nowadays.

I've had remote control cars off and on for years.  With the new LiPo batteries and brushless motors you can really put down some awesome power.

I'm slowly realizing that the key to efficient power is running a low RPM per volt motor (750 in my case) on higher voltage (10S LiPo) runs cooler than the other way around.

It's (hic *) 01:40 hic... here.. Yeah - I was introduced to LiPos by a Z-900 racing mate running a full-loss CDI ignition (and having the young-uns on their whatever newbies think wth?!) He explained their flexibility (literally - mold them around your seat or frame etc) and their energy density & delivery. Later I saw how greenhouse-gassing model aeroplanes had become electric engined prop & jet craft with 300-400A motors! (Alas no less greenhouse gasses here - we don't have nukes. But some assure us (morons?) that we can have "clean coal". rotflmfho..)       As to low RPMs.... If its alternator output, it needs to match your normal RPMs (hence racers tend to gear down). If it's knowledgeable people like me that realise it's all about toque (and not power), then that suggests low RPM too (I loved beating hot cars & V8s with my 4 cylinders chugging along at 2,000 - 3,000 RPM. It made me think of "The World's Fastest Indian" - wasn't that at 4,000 RPM?). But yeah, speed isn't everything. And she agrees. Or to paraphrase earlier - slow is cool, and effective. (Not hot.)

How would I wire up a 3 position switch to off, on, and auto.   I would need a DPDT right?  Auto would be silent, on would make a ground, and off would break a ground.  The grounds in question would be connected to 85 on the fan relay.

Might I impose on you to draw that using the 6 pins on the back of the switch and not the scientific diagram symbols?

There's no might about it... (lol). You draw the back of the switch & I'll add the rest.