Someone please help...

I have four speakers in my car and want to amplify the signal to the
rear speakers only. The obvious thing to do is get a two channel amplifier
and simply connect it up.

However, my Head Unit (JVC KD-SH99R) has a built in amp, and it states
in the manual, if you connect the unit to an external amp, you should switch
the built-in amp off. If I do this however, I will loose the sound signal to
the front speakers, as they wont be connected to the external amplifier.

Is it possible to leave the built-in amplifier on (to get sound to the
front speakers), and still connect the rear line-out connections on my Head
Unit to an external amp, to get amplified sound from the rear speakers?

Will leaving the built-in amp on effect the signal sent (via RCA
leads) to the external amplifier in any way?

Will the Head Unit even give out a signal to the line-outs when the
built-in amp is on?

Thanks for any help