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I think I have a good idea what is going on. It has to do with installing new electronics in older cars. The wiring is different because there was no such thing as memory preset in an older radio. It seems like you are getting power to the HU as you pass the key past the accessory position, but for some reason, this is not true switched power.
Get yourself a volt meter. You can tap into the cigarette lighter or at the fuse block. You will want to find a wire that has constant 12 volts (12 volts all the time, even with the car off and key out of the ignition, like a cigarette lighter). Hook that wire to the yellow lead on your JVC. You will then need to find a wire that has power only when the key is in the accessory position and or the car is running. Hook that wire to the red lead on your JVC. The last wire is a ground, hook it to the black wire. Once you get the HU to behave properly, then move on to your speaker connections. If that doesn't work, you have a HU problem. You can bench test it. Hook both the red and yellow leads to the + side of a battery and the black to ground. It should turn on and off with the switch. With the HU turned on, remove the red wire from the battery, but leave the yellow and black connected. The HU should turn off.
Thanks Lars for the help. It works great the wire that I thought was supplying a constant current was wrong. I tapped into the wire for the cigarette lighter and everything started working just like it was supposed to.
I was going to suggest something similar. Glad you got your problem solved, and now I have something to refer to when I put the Alpine in my 69. Now...if some folks would just make adapter harnesses for older Vettes to hook up to newer stereos without cutting up brand new 400 dollar dash harnesses. :smash: