I have at least spent about 3 hours on this now to search past newsgroup
posts and websites through google, but seems there is no clear answer yet.

I have a grand cherokee limited 97 with infinity gold system. I need to
replace the head unit and speakers, but would love to keep the factory amp.

From what I see, the factory amp requires speaker level input. but most
aftermarket head units assume you use RCA pre-amp output. therefore, I saw
some people said that you would either by pass the factory amp, or buy some
device that converts the signal level. I indeed found one such converter,
but sells for $80 which is ridiculous.

however, I found from the instruction of soundgate.com, you don't need any
interface device, just wire the aftermarket head unit speaker level signal
to the amp.

I assume that no matter what, the connection from amp to speakers would be
just the same, either you use factory speakers or aftermarket ones.

which is the correct answer?

in my case, I simply assume the second one is correct, and wired up the head
unit speaker output directly to amp. the front speakers sound ok. but the
rear speakers barely make any sound at all. I am not sure it is because
something wrong from head unit to the amp or the amp to speakers.

thanks

Hansong