Think you just answered your own question, a good charge should bring it back, the electric heated seats will be drawing a lot of current, whats the voltage reading when you're driving (charging). The cold weather doesn't help either, especially when you are doing such short journeys and then leave it for 3/4 days. Might be worth getting a trickle charger or if you can't hook that up to the car then take it for a run every couple of days if you are going to let it stand for 3/4 days. Warmer weather and longer days are not far away (hopefully) so you may get away with it this year but you'll probably need a new battery next winter.