I am having trouble finding a PMS "Y" cable for the Mitel Express Messenger? Can anyone tell me where I could find a wiring diagram - I will probably have to make one up myself.

Why do you need Y cable????

There is no 'official' Y cable for this. When I've needed to do this, this is what I used:

https://bb-europe.info/bb-euro/literature/232CDT_4502.pdf

I believe the "Y" cable (Mitel PN 50000320) allows access to the serial port for the Express manager connection, as well as the PMS connection.

You can do that, but you have to dial into the EX Msngr and change the port type when you want to access

I don't think there is a y cable, and why have one? One on site, can unplug and hook direct and off site can access via modem.

While Dane is correct about disabling the PMS operation, with a y cable the pms may still try to send info, thats sure to mess something up IMHO.

The Mitel cable mentioned, from what I've read is only a low profile single ended cable to allow the door to be closed.