is one of the go-to sources around here regarding upgrades. That said, I'll clear up a little misinformation: The rears apparently are the same size as the tweeters in the front, but they implausibly operate as full-range speakers. The midranges are technically 3.5 inchers, but most 4-inch drivers will fit.
All 718s share the same speaker locations as the 981:
: In the left and right A/C vent nacelles just behind the vent itself. 3/4" is the technical size; many 1" domes will fit with little modification.
: In the doors near the door pull handle. 3.5" is the technical size; most 4" drivers will fit with little modification unless they are more than 2" deep.
: In the doors, forward and down from the midranges. 7" is the technical size, but many shallow-mount 8" drivers will fit with little modification.
In the 718 Cayman:
: Directly below the rear side window. Size is the same size as the tweeters, but there is considerable depth to work with -- a larger speaker can fit here easily with some modification. These are full-range transducers in the SPP, and two-way components (tweeter, small woofer) in the Bose and Burmester upgrades.
In the 718 Boxster:
: Directly behind the rear seats. Size should be the same as in the tweeters, though function is not.
In any 718 with the Bose or Burmester upgrade:
: Ported transducer type mounted under the front dash directly behind the ignition key/tab, to the left of the steering wheel.
: Integrated op-amp chipset inside the head unit; analog outputs (This is why it's generally the best for aftermarket upgrades)
: MOST 150 bus-controlled fiber optic digital amp module, mounted under the passenger seat -- a location that's empty in SPP-equipped cars, making it a good location to consider for smaller aftermarket amps/DSP units.