This information is primarily for the Bose system available for the RX-8, however the tweeter removal may be similar for the stock tweeter setup. You should start with what is shown in Figure 1.
Removal of the entire plastic plate can be done by hand and is quite easy (thanks to Spin9k on rx8forum.com for first pointing this out). The plate is held in place by two plastic snap connectors. The positions of these connectors are indicated in Figure 2. The easiest method of removal is to grasp the plate on the side facing the window and to pull it away top first (disconnecting the top blue connector first).
With the plastic plate removed, we see the back of the tweeter and the wires connecting it into the audio system (Figure 3). The wires are covered in foam (probably to reduce rattling) and the middle foam segment actually contains a connector.
To disconnect the tweeter, a release on the connector must be pressed. The release is under the foam so you won't be able to see it, but its location is indicated in Figure 4. Gently press the connector on the indicated side about half way along the connector's length, and the tab should release.
Taking out the two screws on either side of the tweeter allows you to complete the removal. Figure 5 shows the tweeter. It seems to be a fairly typical two inch cone driver. The cone and surround materials seem adequate but nothing incredible. On the other hand, the motor magnet is quite substantial for a driver of this size. My biggest problem with this driver is that even the best 2" driver exhibits way too much beaming to be used in any other way than precisely aligned with the listening location (which is almost an impossibility in the car environment). I'm looking to replace the 2" cone with a 1" dome with much greater dispersion. As we can see, Bose has used a single inline capacitor which serves as a first order high pass crossover.
With the tweeter removed, we get a clear view of the plastic plate (Figure 6). The size and shape of this plate is important for those interested in tweeter replacement. See the info section below for detailed measurements.
I measured a DC impedance of 4.1 Ohms using a hand held multi-meter. This is definitely a good thing as it will be much easier to find a replacement 4 ohm tweeter than a non-standard 2 or 3 ohm driver. Also it may mean that the other drivers in the system have standard impedance as well. Actually, matching impedance is not important, matching sensitivity is. In other words, the same input voltage should produce the same output SPL. In order to accomplish this, it will be necessary to select a replacement driver with a voltage efficiency equal to or greater than that of the Bose 2". A replacement tweeter with greater efficiency can be made to match by using an l-pad to reduce the output level. I'm hoping to do a sensitivity measurement soon to complete the picture.
The given the measured impedance of 4.1 Ohms and the 22 uF capacitor, we end up with a crossover point of about 1.76 KHz. The actual crossover point might be somewhat lower due to additional reactive impedance around the tweeter resonance point, but a full complex impedance measurement would be needed to determine that. The good news is that 1.7 KHz isn't too low for some 1" tweeters to manage. Many 1" tweeters start falling off at 2 KHz or higher, so care should be made when selecting a replacement.
I measured several dimensions of the plastic plate in order to determine which possible replacement tweeters could be easily mounted. All the measurements were made to the nearest millimeter but aren't guaranteed to be exact. They are as accurate as I could manage in the small irregular space. Refer to Figure 6 to get a visual reference to better understand these measurements.