Here ya go! Just got it myself!
***To add the NAV/GPS button to the Autopilot window, replace the text for [window03] with the text below. Note that this is for the 727-100 (passenger); the others will have a slightly different gauge00= line, so copy and paste the gauge00 line from what was there:
[Window03] Background_color=0,0,0 size_mm=427,130 position=2 visible=0 ident=10005 gauge00=Captain_Sim.p721.P12!P12_001_ap, 0,0, 427,106 gauge01=Cessna!Nav_GPS_Annunciator, 400, 106, 24, 22 gauge02=Cessna!Nav GPS Switch, 376, 106, 24, 22
or:
***To add the button to the GPS popup window, replace the text for [Window05] with the text below (will work on all versions):
[Window05] size_mm=456,403 window_size=0.4 position=8 background_color=0,0,0 visible=0 ident=GPS_PANEL gauge00=fs9gps!gps_500,0,25 gauge01=Cessna!Nav_GPS_Annunciator, 400, 106, 24, 22 gauge02=Cessna!Nav GPS Switch, 376, 106, 24, 22
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