Hi, we've been through this with our labrador actually! He's just reached full grown.
We didn't find a way of securing a dog harness to the back section of the spider (i.e. over the locking cabinets), though we didn't put much thought into it - there may be a way to do it. When 3 of us got in the Spider he had to go in the passengers lap, until we started training him to sit in the footwell, facing the passenger (i.e. backwards). This is relatively safe and comfy as he can sit between the passengers legs and rest his head in your lap (which puppies like to do anyway).
If your pup is still small enough (and I think he is) he should also be able to curl up on the floor under the passengers legs, as there is a lot of leg room in the Brera and Spider front cabin. Of course getting them used to this is the tricky bit, I don't know about yours but ours loves to see where we are going!
Our lab does much better in the Brera nowhe is full grown, he is harnessed to a rear seat belt but lies along the centre of the rear seat and the transmission tunnel, with his head and front paws on the cooler/arm rest between the front seats - very cuddly!
Brat Pack no.8