Keep the modem and GPS running ... just install it with a lithium battery or wire it directly to a vehicle battery system and place the unit on a school bus or local city transit bus
Or someone could start a small business offering to do this for people ... the business would be to just rent out space on a moving van or transport truck and just fill it with GPS and modem units that have been removed from cars ... for a nominal fee ($1 a week for example), you keep all these units powered and just have the truck driven around everywhere working as a regular delivery vehicle.