We found that the short stubby antennas found on most USB modems were just not good enough (particularly when placed in the passenger footwell of the car). Mounting the box behind the passenger/driver seat is a possibility, but requires laying cable and makes the whole unit vulnerable to theft.
The solution we found is a short flat adhesive antenna specifically designed for in-vehicle applications. The cable runs through the fuse box area of the car, and down to the passenger foot-well. It is not too difficult to lay this (once you know how).