Installing subwoofers on factory head unit?

I have a Mercury Sable 03 LS Premium with a factory head unit. I can't put an after market head unit due to the climate control which is attached. I'm looking to install a few subs in the back and I have heard that I could splice into the speakers from the back into an amp with speaker wire inputs. However my amp that I had in my 99 Jetta does not have speaker wire inputs. There is a converter which I assume I would splice into the speakers, feed the wires into the adapter which converts the signal into a preamp signal to be converted over to the rca jacks. One thing I'm confused about is the remote wire which turns the amp on and off. Where exactly would get this remote wire from inside my car?? And if anyone knows any other alternate ways around this problem please let me know! Any input is appreciated! Thank you in advance.

-drfarkInstalling subwoofers on factory head unit?
The remote wire is going to be the blue wire behind your radio. Connect it to the REM connection on your amp.Installing subwoofers on factory head unit?
That car sux for installing head units. The converter thing you are looking for is a LOC ( line out converter) and it is what you said, speaker level inputs to rca out. .. the remote wire can be wired to anything in the car that turns on with the ignition ( can be at the ignition, can connect to the factory radios ign. wire.) if you want you can wire it to the battery and put a switch on the line. or you can wire it to the fuse box.