Cadillac Audio, Video and Security Systems Discussion, I give up! AMP bypass instructions? in Item Specific Cadillac Discussion; OK so I tried the 3rd gmos-06 and it still doesn't work properly! So I guess the next thing is ...
I never tried re doing mine because it sounded so good. You should have tried to get a factory dvd nav. Cadillac STS 2003 knows how to re wire it to fit into the 98-01 sevilles. All that would need to do is the dealer would have to reprogram the theft lock on the new unit. In answer to your question though I would think 6 1/2 speakers would fit in the doors but up front I would recomend using components.
As for the sizes of the speakers, there are 3 2.5" highs on the dash, 8s in the front doors, 6.5s in the rear doors (but aftermarket 6.5s won't bolt in) and the sub in the rear deck is similar in size to an aftermarket 12". You'd basically have to make custom adapters for everything if you want to replace the speakers. Plus, there are very few aftermarket setups that incorporate a center channel (the speaker in the middle of the dash).
This is true and why I chose not to change mine. I just added satelite radio and am fine with what I have. I may add subs though to the bose system though I know it will never sound as good as it would hooked up to an aftermarket head unit.
Is your 99 sls a Head unit going to a self amplified Bose setup? If It is the same as My 93 seville you do NOT have to rip everything out as people like to tell you. I posted about this before. It was something like "Head unit integration success". And I'm glad to say it still sounds GREAT! right down to the Best of Kansas cd I was just listening to. Very crisp and clean.
Good luck I hope it is the same for you