On the trimble system there is a hydaulic "junction block" mounted next to the AP valve block.  The the overide sensor is mounted in that junction block (I don't know if that is the correct term or not).

The SPX3185 is a very temperamental system to tune.  There were 2 other systems installed in my area at the same time my system was installed.  The other 2 were removed due to poor performance.  The 3185 must be adjusted as the oil temperature changes.  A .5 change in the gain settings will make a huge difference in the performance of the steering system.