The last time I was at my dealer, they connected a Tech 2 to a 2005 H2 SUT to read the part number of the TNR, which displayed part number 15120706.
I doubt you will be able to use a splitter to use 2 GPS devices (OnStar and NAV) with just the one OnStar antenna.
Delphi told me that the NAV unit antenna is an "active" unit.
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