The issue for you sounds like getting from the forward or rear module to the center module. OTOH it might be easirer to put the base station one of the compartments above the front window on an SL. You have a 12vdc supply there also (street side compartment).
For exterior wiring I use
wiring channels from Home Depot or Lowes.
Have a repeater to get from house to TM enclave in back yard but use a MiFi hub on the road.
Why do you need GMRS and not say FRS ? Need a range in miles ?