adding an amp to factory stereo?

Hi, I have an '05 nissan sentra with a mid level factory system. The system has a 6.5" factory sub on the rear deck and a small amp to power that. The wires on the harness of the factory amp are as follows.

blue: 12v const.
blue/white: 12v only when radio is on
black: ground

4 other wires: signal inputs for the amp.

My problem is that I assumed that the blue/white wire is the factory amp remote turn on wire but when I connect that to my amp turn on post, I get nothing. If I connect the blue wire, it turns on but since it's constant, it wont turn off.

Any Ideas why the blue/white wire doesn't work? I tested the current draw on the blue wire when connected to the turl on post... it read about 12mA. When I connected the blue/white wire as a turn on lead, the voltage dropped to 6v. I have no clue.

Thanks alot.

You may need to use a relay for the power turn on wire.

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Yeah, that's what I thought but I'm unsure which one to use since this is such a low power application.

The blue/white wire appears to only be able to put out a few mA before the voltage drops and automotive relays take about 200mA to switch.

thanks for the fast reply. Any other help?

Find any other 12 V switched wire in the car for use as the amp turn on wire.  Or you can use a signal-detecting device if you want to have the amp off when the deck is off.  I just use a 12 V switched source in my personal car because the deck is almost always on when anyone's driving, anyway.  And even if you have long silences while driving, the amp won't be doing anything without a signal anyway.  It's not like it's going to wear out by being on.

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Thanks for the help guys, I ended up at Fry's looking at the relays...

Most of them were huge for 20+ amps. Then I found one reed relay left so I gave it a try. It's small and it worked perfectly.