While it's not impossible to install a new head unit and speakers and leave the factory amp, it'll be a real bear of an install.
In order to work, the Monsoon amp needs 6v which is provided on the speaker leads. An aftermarket head unit will not provide that. Best to just install a new amp.
You should be able to use the wiring, including the speaker wiring, with no problems.
Check out www.bestkits.com
for the appropriate adapters and wiring harness for Monsoon equipped vehicles. Just enter your vehicle info and it should bring up the required adapters and harnesses to make everything work properly.
There are three ways to connect more than one speaker to a single channel: 1) Parallel - both speaker +'s and -'s wired together - load impedance will be halved (2 4-ohm speakers in parallel equal a 2-ohm load). 2) Series - The + of one speaker wired to the - on the other speaker and the remaining + and - going to the channel output - load impedance will be doubled (2 4-ohm speakers wired in series equal an 8-ohm load). 3) Using a crossover - connect the bass speaker (woofer) and the input of a high-pass filter directly to the channel output and the treble (tweeter) speaker to the output side of the filter. In any of these connections, you want to make sure that the load (ohms) your amp "sees" is not less than it is rated for. Most amps will work into a 4-ohm or a 2-ohm load with no problems. Going below 2-ohms is bad, because it will cause the amp to overheat and possible fail. Going above 4-ohms is no problem for the amp, but it won't produce as much power (watts) at the higher impedance and your speakers will not be as loud as they could be.
Hope this helps. If it does, please rate as "FixYa:. Thanks!