Head units remote wire

Are you saying that your 12volt remote turnon (usually the blue wire from the headunit) is not sending a (+) 12V to the amp?  Unfortunetly if the head unit is not sending the 12V you may need to have it serviced.  You may be able to use another (+)12V output from the head unit to turn the amps on.  Most headunits have a wire to activate a radio antenna.  In most cases it is not used because the majority of cars to not have a motorized antenna.  You should be able to use this wire.  I can't remember though if it activates when you turn AM/FM and not when you turn CD on.  Of course you want your amp to turn on in either case.  Use your wire tester to make sure that you get a +12V whenever the radio is turned on period.  If this does not work you will be able to find a 12V signal elsewhere in the car.  Lets save this for plan B.

You could connect to the red wire behind the radio, but if the unit is under warranty I'd get it fixed.  If you do use the red wire, your amp would stay on whenever the key was on, regardless of the radio, so you could put an inline switch like you asked.