Many people have said to just let your equipment warm up for 10 min or so in below freezing temps, but what if your sub is in your trunk, then whats the rule of thumb?
I live in Alaska and it hits -40F for at least a few weeks each winter. I do not know of any standard for this type of thing. That said I would not start my car and slide in the Telarc CD on the coldest day of the year for a SPL test. I warm my car up before driving, and start playing music at a low level, and gradually increase it over about a 5 min span of time. The only major problem I can remember was back in the mid 90's and people were Armorall'n there subs and surrounds. Some of the surrounds soaked up the armorall. Then froze. Then cracked. Then you go to the store to get new subs.
I've also had problems with the head unit getting warmer, faster then the disk. The disk will cease to play. Take out the disk and it will be cold and have a fog on it. Put it over the heater vent for 10 seconds or so, and no more problems.
None of my current rides have a trunk (suburban, Subaru legacy wagon, civic hatch, reg cab truck) but I have had cars with subs in the trunk. Ive never had problems with there either. Playing them at a low level will help to heat up the voice coil and motor structure, it will also warm up the surround (slightly). If your worried then go out on a cold day and feel the surround, if it feels to stiff don’t play it I guess. But I think manufactures take this into account when designing speakers in general.