1995 Subaru Legacy amp install

I'm trying to install an amp and subs in my Legacy which has the factory stereo with tape/cd. It has a power antenna that turns on when the radio is on, but turns off when the cd player is running. So in turn my amp shuts off when i play a cd. there are 3 wires leading to the pwr antenna- ground, switched, constant. I first connected it to the switched, but that was the one that turns off when you play a cd. I hooked it up to the constant, but then i found out the hard way that it doesn't ever shut off, even when the car is off, and you get a dead battery the next morning. 

I read that a early 90's model Mazda did the same thing, and it said something about installing a relay. But I haven't had to install one of those before. What kind would I need and how would I do it? Thanks

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Jason

You may want to try the positive on the cigarette light (in most japanese cars it's a switched acc wires).  That's what I did in my 97 altima.  Or you may just want to get a digital multimeter and check each wire in your radio harness plug and CAREFULLY  splice into it or use a wire tap.  Here's a wiring diagram that might help.  I'm assuming you're trying to find a way to connect your remote turn on wire on your amp? 

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Duane...

If you think you're confused, imagine how you feel.