Navigator Speakers Cut Out

I pray someone can help me with this!  I installed an MTX amp and sub in my 2005 Lincoln Navigator.  The factory stereo does not have RCA outs, so I used speaker-level input into the amp.  When I turn on the car, the amp does not turn on, and the factory speakers don't work - but the factory sub does!?  If I  unplug the speaker level input from the amp (with the car still running), the factory speakers turn on!  Then...if I plug the speaker inputs back into the amp, all speakers work, including the amp and the sub!  Everything works perfect.  So here's the catch, when I turn the car off, then back on again, the same problem happens all over again!  I need to unplug the speaker inputs to the amp, and voila', the factory speakers turn back on, then I need to re-plug in the amp again.

Has this every happened to anyone else?  What is causing this - and more importantly - how do I fix it?  Any help would be so greatly appreciated.

Thanks - Tom

I don't know why, but for some reason I listed my truck as a 2005 - its not - its a 2000.  It does not have the THX/Nav system.  I tried splicing the input wires from the factory amp to my MTX amp, using the RCA inputs, but the input level was way too low.  As a last resort, I tried taking the speaker wires from the factory sub amp and connecting them to my MTX sub enclosure and that worked the best.  Problem is, I can't figure out which wire is positive, and which is negative, and I don't want to blow the sub up by running it in reverse.  I'm mega-disapointed that I can't seem to get my MTX amp to work.  Its 450 watts and it thumps when its working right.  I would throw out the factory head unit and get a good aftermarket unit, but the factory unit has steering wheel controls and a 6-disk CD changer, all of which are integrated.  At a minimum, I'd be happy for now if I could figure out which of the factory amp speaker wires is positive, and which is negative.  Thanks for all of your help - it is much appreciated...

Tom

See my answer to this in https://www.the12volt.com/installbay/forum_posts.asp?tid=55388&PN=1

DYohn-

I ordered the LOC you recommended from Crutchfield.  I should have it tomorrow and I am going to see if I can get it to work.  Now, the last thing I will need to do to solve the puzzle is to figure out where I can find an accessory 12v wire to power the amp's remote on.  Of course, the factory HU does not have a remote out, so that's going to be a challenge as well.  Thanks for all of your help and I'll let you know how it goes tomorrow...

jstruckman - that is EXACTLY what I needed!  Thank you.  The LOC worked like a charm - even had output levels to balance the output of the sub vs. the factory speakers.  All I need to do now is to hook up the TRIGHOHD and I will be good to go!  Thanks everyone - this has been fantastic!