i have a 2005 Bonneville GXP, came stock with the Monsoon system.
changing to an aftermarket wasnt so simple. but i can definitely tell you it was worth it
most aftermarket head units will sounds better than sound system even on stock speakers.
i can help you with everything if you wanna install an aftermarket HU, just let me know
you will need a special harness if you're willing to keep onstar and hands free calling, also another harness to keep the steering wheel controls working.
you can pick any headunit you want, single or double din, i have the JVC KW-NT1, and i can tell you, best HU i ever touched, what makes it so amazing, is how fast, responsive and simple to use. you can browse thru all the music on your ipod or SD card or USB stick without any loading no matter what.
as for the harness there are two that i am aware of. the PAC harness and the Metra harness..
if you buy the PAC (not sure of the exact model at the moment)you will lose the onstar feature, hands free calling and the chiming comes out of the harness instead of speakers, and your system will sounds like ****.. JMO. i tried it before and for a second i did regret going from stock to after market
if you buy the metra GMOS-09. everything will remain like stock and you wont even know you touched the car. onstar still works, so does the hands free calling. and also all the warnings will remain the same (low fuel or check gauges etc).
as for the steering wheel controls, you will need the PAC harness, they are prolly the best in the market when it comes to steering wheel control harnesses. model number depends on the HU choosen. MAKE SURE the head unit you buy have a rear input jack for remote control other wise you cant get the steering wheel controls to work.
as to answer your question. I'd say go aftermarket
any other questions feels free to post here or send me a msg
hope this helps
2005 Phantom GXP - Stockish