The next step would be to call Van's and ask them to walk you thru the wiring diagram. You will have to ask them for the particulars on the GPS data wire. Then perform continuity checks for the GPS data wire from the GPS to the switch panel and then from the switch panel to the D180. During the continuity checks ensure the wire did not break off of the pin and the pin is tight in the connector.
None of us can help here as a wiring diagram of the switch panel is not available.
Assuming you did NOT change any of the default settings in the GPS or the D180, other than the one mentioned by Pete which has to be changed from FAST to NORMAL. If you did, have them walk thru all settings in the GPS and the D180 related to communication between the two.