assuming the key worked smoothly in warmer weather

then i would use any lock deicer

be liberal with this stuff

700's had a bad design for handle/latch assemblies they used plastic pieces

in them and they are proned to binding after time and as a result the only fix is to remove the assemblies (assuming the fasteners will break loose) and clean them

then manually get the free movement back in these pieces and soak them in transmission fluid

reassmble and KEEP them well soaked in transmission oil

when below freezing try to not wash the car, especially using car washes

hope this is of help

kgb