The rear speakers are really just for "fill" like in a home ent system. They won't blow you away, they just help "pull" the sound stage back towards you. Otherwise, it's front heavy and focused just under/behind the windshield.
Regarding the rear speakers, it sounds like the PO just wire them off the h/u. You can double check this by looking at the larger of the 2 connections on the amp. I'll bet there are 4 pin slots empty (5, 15, 6, and 16).
Your amp, in the front trunk from the factory is a 4ch by 40W (4x40). It puts out 40W max, the head unit has built in amp with 18W max. Thus why the system sounds unbalanced. Plus the rears are only 3" or 3.5" (OEM rear speaker kits were 3", some aftermkt providers made 3.5" rears). You can transplant a 6x40W amp from a 996 into your system to power the rears. This would require getting an amp w/ the same architecture as yours (either M490 or M680 - sticker on amp or option sitcker under front hood will tell you which you have - plus the M680 has a DSP controller just above the batwing in the center console). Then you'd have to pull the h/u and disconnect the rears and run an extension to the front trunk amp. PNP, an aftermkt rear speaker kit provider, sells/has the extensions.
Regarding the fading, etc., the Dash speakers are considered the front channels, the rear speakers are rear channel. The door speakers are driven by combining the front and rear channels. Thus, if you fade either all the way front, they almost completely cut-out, same for rear. Door speakers are only mid (semi-low) as there's a filter built into the amplifier.
Speakers by locations:
dash - components (4" and a .75" tweeter) - 4 Ohm
door - 5.25" woofer (using the term loosely) - 2Ohm
rear - either 3" or 3.25" - one pair per side should be wired in parallel 4 Ohm. If wired in series, you can get a little more out of them by switching to parallel.
A lot of people, me included, have upgraded the speakers in the car. Although you are happy w/ them thus far, you'll find they really lack clarity and ooophm. Lots of details are in the DIY section here. Also, it can be difficult to procure a 6x40W amp. There are lots for sale, but IMHO, anything over $50-75 you'll be better off just going aftermkt w/ an amp that fits your needs.
Hope I answered all your questions. If not, plenty of info in the DIY section - just search on speakers or amp ugrade, etc. Good luck
