I didn't take any pictures when i installed the new speakers, so i will try to describe it in words:
In order to replace the stock speakers in the doors, I carefully pryed off the door panels from the bottom to the top. They come out a lot easier when you have a little plastic fastener removal tool (basically a little fork shaped pry bar that you can buy at auto supply shops). Note that you have to remove the screw holding the door handle and screw holding the inside door release (which are also hidden behind plastic pieces). This process was very similar to removing the door panels on my old 2006 mazda6, which I found instructions for. I wouldn't have tried removing the door panels on my new cx5 without having practiced on the mazda6 first.
I don't think any larger speakers than 6.5inch would fit in the doors. I cannot vouch for the size of the tweeters that are in the top of the dash by the A pillars because I only have the GX/manual transmission (I believe this is the 'sport' trim level in the US), and it doesn't have the tweeters.
All in all, installing the Alpine SPS-610s was a huge improvement in the sound - much clearer than the $0.05 paper speakers that Mazda puts in these cars.
The stock speakers come in a plastic basket that mounts into the door metal inner panel, and the basket contains the other end of the connector. So, what I did was rip out the stock speaker from the basket (I don't ever plan on putting the stock speakers back), fastening the new SPS-610s into the basket using self-tapping screws, and then soldering the wires from the new speakers into the connector in the basket. The assembled SPS-610/basket assembly goes back onto the door metal inner panel using the existing bolts, holes, etc. So now I have a pretty much "plug and play" system - the new speakers are set into the baskets from the old speakers, and since I kept the connector from the old speaker baskets, I can bolt/unbolt and connect/disconnect the speakers if I need to.
Now I really wish I had taken picture when I did the install. Oh well, next install I'll take pictures
Oh.... I would very much be interested in knowing if the Polk Audio db651 fit. The depth will not be an issue since they are about the same depth as the SPS-610, its the massive tweeter on the db651 that sticks out like .5-1" outward, which is why they provide custom grills with the speakers. db651s, the slim tweeter version, fill it with stock grills, but mid bass and highs are better on the db651.
Let us know!
I was looking at the front 3.5" dashboard speaker connectors and brainstorming what I already had to use without building a custom cable.
Guess what, these little motherboard 3 pin fan extension connectors work perfectly!
Their about $1-$2 a piece on ebay, one seller has (5) for $3.67 shipped.
Two of the three pins fit right into the speaker connector while still attached to the white bracket, the spacing between the pins is right with the last pin sits outside the speaker connector. All together, its a snug / secure fit and you just need to crimp some blade connectors on the other side of the cable and your done.
On the speaker side, I've read reviews on the polk db651 and the main negative seems to be the lack of low bass. So they might have been designed with the idea that a dedicated sub would be in place as well. Going to research some alternatives and maybe head down to best buy or another car audio place to listen to a some before buying.