would my 2 guinea pigs be ok in the garage/shed ??

There isn’t heating….
my car is kept in the garage weed killer and windscreen wash etc is kept in the shed, they’d be kept in an indoor cage. They have blankets etc food and water.
????

Never keep a guinea pig in a used garrage with a car as the fumes are not good at all. I would move any toxic things, such as weed killer and keep them in the shed. Ideally in an outdoor cage as the offer more protection from the cold. If not, make sure they have plenty of bedding. A heat warming plate is also a good idea, you can microwave them and they stay warm for up for 12 hours, and can be put under the bedding.
The pets at home website and store sells them

In an ideal world you should really wait until summer to put them outside, especially in the indoor cage!! Otherwise just make sure they’re nice and warm, and take out anthing that could hurt them!