Mounting the antenna sets up the transmission pattern for the vehicle.
Mounted in the rear the transmit pattern will go toward the front, if mounted to one side the pattern will be toward to opposite side of the antenna.
Also, mounting in the rear tend to bring in fuel pump noise, toward the front brings in engine noise, everything in a CB system is a compromise so the mounting position needs to be first where you wanted it and second where is matches best without a lot of noise.
there are Quick Disconnects (QD) to remove the antenna before going into areas with low overhead clearance. These QDs do cause some matching problems but most can be overcome.
Mounting the antenna on the bed rails and/or mirrors can cause interference with the body of the vehicle reducing the overall performance or transmitting distance.
Dual Antennas are a thought but you need at least 1/2 wave length between the antennas to get the best performance and distance, i.e. 1/2 wave length for the 11 meter band for CBs is 18 feet. The 6 feet you will get on a pickup will not work well. Also when tuning dual antenna you need a Co-Phased Harness and you will need to tune each antenna using a piece of 50 ohm coax, once tuned you need to add the Co-Phased Coax Harness which attaches the antennas to the CB.
Still the best location for an Antenna is on the roof centered on the vehicle if possible, the higher the better.
Luck,