Stereo Wiring

I am installing a stereo in my 84 Cabriolet/Rabbit. I know that factory VW did not come with a car radio acc switch on the ignition wiring. I know that the factory car stereos for VW's were hard wired in directly so you could turn them on even when the car was turned off. Not a bad idea as long as you have the turn off knob.

I found a wire from my rear defrost sensor that has power once I turn the key to the first click and it turns off with the key. Can I Splice this wire to my red wire on the stereo? The yellow wire will still go to a constant power supply of it's own but can I share the other wire for both? Or do I have to not hook up the rear defrost to put this wire to the red wire for my stereo?

Thanks