Mazda had a couple different radios, even between the series. Some had a graphic equalizer and were different than the ones without. I sold the S5 radio without the equalizer a while back, but I am pretty sure that the connections on the back were different.
I have scrapped several S4s and S5s, so I just keep the series together.
Part number lookup is one way to tell compatibility (at the dealer).
I can't find it now, but there used to be a parts manual for each series on the internet that you could access to cross-reference.
You can also go to NAPA, etc. and see if they list the same part number for both series (works better for GM/Ford/Chrysler than for Mazda).
I wish I could help more.
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1986 GXL ('87 4-port NA - Haltech E8, LS2 Coils. Defined Autoworks Headers, Dual 2.5" Exhaust (Dual Superflow, dBX mufflers)
1991 Coupe (KYB AGX Shocks, Eibach lowering springs, RB exhaust, Stock and Automatic)
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