Need help with rear defroster circuitry

[GTP]

I am considering tapping into my rear defroster circuit for a DIY differential cooler build.
This involves the interior, driveline, and is needed for the track. So I am posting in the General forum hoping for more views, haha.

My idea is

• Leverage the rear defroster circuit, which comes from 40A Fuse# 9 in the engine bay.
• Cut into the hot line and install 2-way switch.
• Run second hot line to thermal switch and 7.5A fuse for the pump underneath.

Operation is A) switch flipped to original defrost circuit during cold months, or B) switch flipped to diff cooler for warm months.

What I want help with is 1) circuit diagram, and 2) where/how the defrost wires run.

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[Jaymar]

I guess I'm missing the point to deactivating the rear defrost to activate the diff cooler? If you are going to introduce the complexity of switching between the two as opposed to just adding an additional circuit for the diff cooler what is there to gain?

 

[GTP]

You may be right. It might be just as easy to wire in all new wiring from an unused fusebox circuit as to do what I suggested.

 

[ORRadtech]

It's pretty easy to run a dedicated power line from the front fuse box or even the battery. Coming through the fender into the interior, under the carpet/kick plate along the existing harness and behind the side trim, is all pretty easy and you're right under the back seat. You can fuse tap into the fuse box without damaging or altering any existing wiring or risking blowing a fuse for something else.
That's what I did for a small amp and don't think it took more than a couple hours total.

 

[GTP]

Duh, I guess I could use Relay #90 in the engine bay fusebox, labeled Mach 1 Differential cooler.

The relay is not there, and so the wiring may not be also.

 

[StangTime]

You might not want to do this. The window defroster is a timed circuit that will automatically shut off after a while. Is that going to be a problem for your diff cooler? Will you remember to turn it back on when you're doing laps?
I would just wire in a new circuit with a dedicated toggle switch or have it live all the time with thermostat control. That way you won't need to even think about it's operation.

 

[GTP]

I'll probably just install this.

https://www.summitracing.com/parts/sum-890023

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