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Just guessing that you have a 944/2, since you didn't say... but 490 is "sound system" and 494 is "additional amplifier". No help there. Is there a manual that came with the car? The audio manuals are the best bet.
It shouldn't be too hard to narrow down. Do you have the window sticker or separate build sheet? My '89 N/A had a Reno SQR46 w/ the standard 4 speaker setup.