*Note: These topics will expand greatly in the near future*
Most people start upgrading their stereo for one of the following reasons:
It is very common to see a broken cassette mechanism on 20 year old stereos. Unfortunately the stock Z31 units are no different. The story is the same for the speakers. It may be from abuse, age, or water damage. Some cars originally came with an external amplifier. These amplifiers are made like most stock automotive electronics. They are pretty durable and don't fail all that often.
Today CD players are the standard format for both stock and aftermarket stereos. They also bring a much better set of tecnologies in signal processing and amplification with them. Currently DVD players are becoming more mainstream and affordable. Most of them are intended to play DVD movies or to read DVD Map disks for navigation units. There is also a format of DVD audio which has not been able to get a foot in the door.
This is the most popular upgrade in car audio. Adding a sub to even a stock stereo can make a HUGE improvement in the overall sound. Since the stock speakers are only 6.5”, they are very lacking in bass reproduction.
Most mounting kits available for the Z31 will accept a single DIN chassis stereo. The maximum depth is about 7.5” although it is rarely an issue.
* Crutchfield sells a mounting kit for that is basically a steel bracket and an abs plastic faceplate. Item number is #176N81.
* Metra is similar but the bracket is made of plastic and the whole kit is desinged to accomodate several types of radio chassis as well as the early cars with an antenna switch. Part number is 99-7411.
* Metra also sells a wire harness that allows you to connect an aftermarket stereo to the factory harness. Part number is 72-9300 (unverified).
Stock Speaker size is 6.5” on BOTH front and rear. Depth is an issue and should be checked before buying speakers.
Actual cutout is 5 1/8” as referenced by this thread.
Thanks to Strike for actually finding a tape measure and going out to his car for real information rather than relying on internet rhetoric.
External amplifiers are very easy to add if you have an aftermarket stereo to go with it. Otherwise you will need either a low-level adapter or an amplifier that accepts speaker level inputs. You MUST run a new power wire for external amplifiers. No factory wire will carry enough current to do the job.
If you added an amplifier already, then the hard part is done. Find a subwoofer setup you like and fits in the space you want to use and go for it.
Anything you can imagine