Sorry for the delay in reply. I finally got a chance to talk to a Whelen tech rep at Fire Show West. The reason behind the expansion module is to work with the negative inputs from the Ford PI, both Utility and Sedan, that are on the steering wheel. The inputs can be used to control various outputs on the CentCom, IE wire an input to the door open wire and turn off the drivers side modules of the lightbar, or turn on ground lumination lights (fire engine). The inputs are factory set to see ground ("low switch" was the term the rep used), but can be programmed to get 12 VDC input.