Need help: Simple wiring question, Sony head unit

[HeelnToe]

I'm putting a new Sony head unit in my 1972 280SE 4.5 to replace the terrible JVC casette unit the PO had installed.

Simple question: the old unit is removed, and I can get the Sony to power up and work, but...

Regarding power, I only have a black and yellow wire in the car, no red. If I attach the red and yellow wire on the stereo to the single yellow wire in the car, the stereo works just fine, but it is always powered on, even when the car is off. Obviously not a good thing.

The yellow wire is soldered onto the fuse box to the first fuse, so I'm guessing it's the "accessory" wire, meant to signal to the radio that the car has been turned off. Am I right about that?

What do I need to do? Anyone have any experience here? Do I need to put in my own red wire?

Thanks!

[donbryce]

The easiest way to find the wire you want is with a 12V test light. Just attach one lead to the best ground you can find, and the other to the end of the wire. You want to find the one that is 'dead' in all key positions, including the key out of the ignition, except when the key is turned to the 'run' position. You could also use a wire that is hot in the ACC position, for listening when the car isn't running but the key is turned to position 1.
A head unit draws very little power, so it might be OK to run it off a wire that wasn't originally intended for the 'radio', and you'd be sharing a fuse with whatever else is on that circuit. No biggie as long as you know what fuse you are on.
How come you can't identify/use the switched power wire the JVC unit was connected to?
This may not apply to you, but for the life of me, I can never understand why people stress over wiring colors and associated wiring diagrams for their particular year/model when installing a radio/CD or whatever. The time spent trying to match wiring diagram colors with the harness in your car would be twice or more the time spent doing a simple test for the power wire you want. BTW, a multimeter set to 12V range will work fine too, but the test light is simpler IMO.
Oh yes, almost forgot, I suspect the yellow wire, which appears to be always 'on', ie., has 12V constant, is for the radio unit internal memory/clock/etc. The wiring chart accompanying your Sony should note this.