I want to turn the head unit on while the car is off.

I tried running a simple two-wire push button switch between the black and red (constant and switched) wires. That didn't work. It didn't do anything. I thought I could just add power to the switched line, and it would pop on. Nope.

Has anybody done this before?

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1982 300CD Petrol/Black Leather

1983/300CD,

What is a head unit? U need to explain what U are trying better to do to get an asnwer.

P E H

a Head unit refers to a stereo head unit. I.e. stereo with CD player etc.

IF you take your aftermarket head unit and run a wire from the battery with a 15A fuse and wire it to the switched lead on your head unit (red wire in most cases) you can have power to the unit.

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91 350SD
14 F150 Eco
19 Fusion Hybrid
11 GT500

So, sometimes it's better to go to your actual installation manuals than to read wiring descriptions in the forum archives.

So, I think my problem is that the constant power is supposed to be YELLOW, not black. The old posts I was reading speak of black being power, and brown being ground. My head unit's installation instructions specify black as ground, and yellow as power.

I guess I was hooking power to ground. Oops.
I better go change that.

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1982 300CD Petrol/Black Leather

I separated the red (switched) power wires where they were connected.

I then connected the red wire from the car (not from head unit) to the far end of a 3-post switch.

From the middle post of the 3-post switch, I connected the red line to the head unit.

The last post connects to the yellow (constant) power source.

I can now switch between "switched" and "constant" power source to the head unit's "switched" power in.

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1982 300CD Petrol/Black Leather