I know there were changes between 88 and 89, but I assumed that a side mirror would be the same? My passenger mirror is broken on my 89 convertible, I saw a used on on ebay for an 88, but wanted to make sure it'd fit.

 

It would probably fit, but the mirrors are different shapes. There are 2 different S4 mirrors (aero and non-aero), and I think S5s just had one version. The S5 mirror shape is more rounded.

 

I bolted an S5 TII mirror on my S4 TII no problem. Unfortunately I found out that they were different when it was too late.

 

Mine are the aero, and these look like aeros. From the pictures they look identical, but maybe the differences are subtle?

 

S4 aero: ImageShack® - Online Photo and Video Hosting

S5 mirror: 89 90 91 MAZDA RX-7 L. SIDE VIEW MIRROR | eBay

If the listing shows the top one, the seller just doesn't know what year it's from.

 

Most of the exterior parts are transferable