If the bolster switches are on the front lower corner of the seat I believe you could possibly assume the model year of the seats from the switch color. If '91 seats I'd think the rocker would be "gray/orange" and if '92'/'93 the rocker would be "black/orange".
This eBay offering could be used for switch location ID for '91 thru '93. The seats are reversed in the presentation, driver is on left(worn upper bolster). A '90 would have everything on the "top" both left and right on each seat.
http://www.ebay.com/itm/1991-ZR1-ZR-...hXPNjq&vxp=mtr
This is typical of the '90 seat. It's a very generic image but you can certainly see the 4 seat control locations.
http://www.ebay.com/itm/1989-1990-Co...5XqO1r&vxp=mtr