First off thanks to stockguy, magik8, and mazdameister for helpful posts.
Wiring diagram (courtesy magik8):
How do I enable the factory sub with an aftermarket radio?
Once all your other speaker stuff is connected there will be an unused wire hanging behind the deck - L/Y (blue with yellow stripe). Splice that into your ACCESSORY wire. It's that simple.
The blue wire with the yellow stripe is the turn-on lead for the subwoofer. The actual 12v power for the subwoofer is supplied through a blue wire with a white stripe. The other 5 wires on the stock subwoofer are ground (black), and leads from the front speakers (blue/orange, blue/black, blue, and blue/green). (The solid blue wire changes color somewhere towards the deck connector...the schematic here may have an error in it.) You shouldn't have to hook anything special up to get the front speakers working. Only the rear speakers work off a separate amplifier, located behind the radio.
Which cars/systems had the subwoofer:
The factory powered subwoofer that came on ALL 1st gen LX, LE, and GT's. DX's did not have the factory subwoofer.
The premium sound option was a 2 DIN receiver, with automatic Tape deck, a 25wX2 rear amp for the back speakers, and the powered sub under the passenger seat.
For 88-89; LX's had the option of the 7 band graphic EQ, standard on GT's.
For 90-92, its nearly the same, but the radio/tape deck is one piece, and there is no rear external amp. GT's came standard with a DIGITAL eq, and LX's had it as an option.
A CD player was option most years.