If you have to drive all winter, it can be done easily. Goodyear & Bridgestone both make great "winter" tires for these cars...just get a set for the rear and be carefull. The F1's are actually an all season tire, but don't really allow the stoping power of a snow tire. If you got the money buy all 4 but don't play hard when it's dry, they don't make spped rating over like 110-130 for obvious reasons.
In 2 previous Mustangs all it took was a good biting snow tire for the rears and they drove fine....stopping is tough though so you need to be cautious in slick stuff. My 91 LX came with a Michelan XGTZ4...those tires were all I needed, no snow's for that car.