Thats entirely up to personal preference and convenience!
Some people place it right under the bonnet/hood of a car and connected to the battery, other people who have access to the 12V electrics inside a car place it hidden behind a panel or the glove compartment. Some cars have the battery in the trunk/boot and so it makes sense to place it there.
An obvious choice for two wheelers with a place under the seat is to place it there, other options are anywhere hidden behind the casing.
It works fine in just about every part of the vehicle as long as it is sitting nicely (stripy side facing up) and well attached with the provided velcro pads. Do not place it directly on the motor because of heat 😁