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