The Remote IMU (RIMU) cabling is not long enough to mount the RIMU outside.
The RIMU (sometimes referred to as a gyro) is not waterproof and is designed to be installed only in the cab.
The RIMU should be mounted flat on the floor on top of the floor pad, typically in the front right corner of the cab.
The RIMU needs to mounted securely with screws into metal. Before drilling, verify the screws will not damage any vehicle components.