I also have a 93' with grey leather. I removed my front seats, and installed some cloth high backs, that I paid $250.00 for out of an 88' GT.
Do you have the lowback seats? If a 93', probably the same as mine. I had quite a few offers on mine, but everybody was to far away to look at them (had them on Corral auction)
Anyway, I think $600 is way to much, but if you find somebody willing to pay it, go for it. I would expect $400 to be a more reasonable price.
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1993 GT/AOD
'93 Mustang GT