stock HU output to oem amp

this is merely a quick solution to a friends lexus 95 300....  or maybe 96.. anyways i think year and model don't matter for this one....  i am adding a sub/amp to a  lexus that already has an aftermarket HU.. a jvc something or other with one pair of RCA outs... the speaker outs are all taped up/not used.. the speaker setup in the car is all factory 6 speakers altogether.  2 tweets up front, 2 mids in front doors and i believe 2 6x9ishes in the rear.. no sub....  YET.. the sub is only going to add some body to the system.. not a competition setup by far....  so the factory speaks and all that pushes them will suffice.. now i gather the oem amp has a built in crossover/high pass for the tweets in the front door and some sort of filtering for rest of setup....  since the oem amp is being fed by the ONE pair of RCA'S (MARKED REAR ON JVC HU which will hopefully grant me control of the sub soon) there is no fade control at all as of now..  can i just feed the amp with the stereo speaker outs (with a hi/lo gain control....) or just wreck the amp, rewire the 2 front door speaks and the 2 rear speaks and kill the tweets.... or .. grrr..   this whole installl is really giving me a headache..  i don't want to re-wire the system, however it would be really nice to have balance/fade control again.,.  what to do....  thx. in advance