Vickers Viscount 800 does not engage GPS

[Paul1]

I have downloaded and installed the VASP Vickers Viscount 800 from page 9 of the Propeller aircraft for FS2004 (together with the guages fix).  

But, when I try to turn on the Nav switch (to engage the autopilot to the GPS) the Heading hold switch keeps resetting itself on.  It seems to force the Nav switch off.  Thus, the aircraft will not engage the GPS.  

How can the aircraft be made to engage the GPS ?

Thanks for any help !!!