so what is the difference between a monsoon setup and a non monsoon setup I have a 2000 Buick century With six speakers I would like to know what the monsoon setup is and can I add it to my car

I am not sure on the specifics, but from what I gather, it'* a seperate amp, with additional speakers. I believe Buicks used "Concert Sound" instead, and never crossed paths with Monsoon.

For the amount of work that it seems may be necessary to switch over, I would be inclined to swing aftermarket/custom myself. Possibly find a Lesabre with the 'concert sound' system and look for the extra speaker grilles (Our PAU had one betwen the defroster vents for example). Steering wheel controls and chimes can be adapted to aftermarket as well, and it ultimately gives better flexibility and expandability. Although, if you want to retain a 'stock' look, it may also be good to look in to which head unit plays better with external amps.

mine has concert sound and steering wheel controls I want a apline cde headunit

You can add monsoon but it requires more then I think you want to do.
Requires changing the complete body wiring harness you need all the speakers head unit the monsoon amp and rear package shelf cover as its different to.
But it is all plug and play the monsoon is a 8 speaker system with tweeters counting as 4 of them

I should have said there is a amp in the monsoon system to in the trunk

Its not worth the hassle. I did this several years ago with my 97 Bonneville. You have to rewire the entire system, and change out the speakers. Factory speakers are 8 ohm and Monshit speakers are 2 ohm.

well so I should do custom with parts from crutchfield can l add another set of tweeters and where would be the best location im guessing the dash I need new tweeters for the front door anyway

If you are ordering from them and going custom. Then get a new HU, amp and ALL new speakers. Get rid of the factory paper-cone garbage.

that was the plan

Don't go cheap. If you want good powerful audio, you have to spend the money.