according to this site: http://www.modifiedlife.com/1989-toyota-celica-car-stereo-wire-diagram/ we have 4" fronts and 5.25" rears. but what i dont know if this is for the hatches or for both....... so im not sure. a 5" midrange woofer was able to fit in the back nicely without any mods. the only thing is that the cover doesnt sit too well with it. broke mine trying to refit it. :banghead
I have 4" all round in my liftback. Instead of replacing the rear ones I'm going with Stig's idea on using ACM - Aluminium Composite Material to build a new parcel shelf with some nice 6x4's. That wiring diagram will be handy when I put in a mechless car stereo system.
A little bit of a necropost...but give me a break, I haven't had a st16x for a long time now. Just confirmed that Fronts are 3.5". Zero modifications, just bolt on. I even reused the foam from the stock speaker because the size was dead on. For the rears I believe it's 4.5". Not quite 5". I bought 4" for the rears and it's too small so I'll take photos later when I redo the rears Here are photos of the fronts: