Considering that I dont know what year your car is... do you have rear pads or shoes? And what condition are they in. I've noticed that if you have drums... and you use cheap brake shoes they tend to lock up a little bit easier in the cold. Also, if your rear tires aren't good it will cause it to slide as well.
__________________ 88 T-Top GT Flowtech Cat less H-Pipe, Flowmaster Original 40's, FRPP A/C Delete Kit, FRPP Smog Pump Delete, Cone Filter (no cold air yet due to lack of inner fender wells), 12000k H.I.D.'s 4 fogs and heads, Alpine CDA-9855, Orion HCCA 10" subs, Rockford BD 1500.1, Orion HP4600 4 channel, All Orion Interiors, March Perf. Adj. Alternator Bracket, Smog Delete, 3gen Alt, Underdrive Crank Pulley, Eibach Sportline Springs.
Carb Swap in progress... Edelbrock 600, Performer RPM Intake. | 
Breaking on snow with mustang. 
