Replacing Rear Speakers - anyone done this?

I have a stock radio in my marquis that has the cassette player, and about a year ago i took it into circuit city and had them put in a 12 cd changer in the trunk, my stock marquis speakers in the back deck are on my list to have replaced, these are 6x9 right?

I have a pair of Polk 690 6x9's coming in out of a wrecked car from my brother in law next week, he said they sounded pretty good in there and they are tearing the car down for parts, so i am buying them for $40 and hoping they will fit and sound better than my stock speakers.

I have never been up in or under that rear panel, are the 97 marquis holding 6x9's and how hard are they to change out, do you swap them from the trunk underneath or from backseat from top?

Thanks for any help on this, and what by chance size are the speakers in the door (are they hard to get to?) might change those too in the hopes the speaker swap out would make a good improvement sound wise.

Curious too what others have thought experience wise swapping out stock speakers for a decent set of speakers, are the marquis stock speakers blah like most car stock speakers?

Bill

Ah I JUST found this tonight here on google, it should help alot for anyone wondering these same questions as I did here..

http://www.autoclinix.com/Mercury/Rear%20Speakers.html