Flyer,
It almost sounds like you have never had the alarm (as opposed it ‘has now stopped working’). If true the previous owner may have removed it. But wait there is more. And frankly I am out of my depth on this one, so others will have to explain.
I have the EFI workshop manual for my 5.0 GXi-E (7742218) so I went looking to see if it contains your 5.7GXI-D and it does. What I found odd is that the diagram does NOT include an alarm. Huh? This manual covers a variety of EFI engines and there are alarms shown in their wiring diagrams. Its items 23 for the 4.3GXi-A that is also for a 5.7GXi-B and others. And as you go thru the other 8 in total schematics (covering dozens of engines), 3 of them do not show an audible alarm.
Now the real fun part. My “E” version is not included in these schematics. Some say was an oversight by VP. Regardless, on another post, someone with an “E” evaluated that its wiring diagram is the same as your 5.7GXI-D. But I can affirm my boat does indeed have an audible alarm that beeps prior to engine start. It’s under the dash. So, I think the ‘wiring diagram’ showing the alarm must be associated with the particular boat maker and that VP provided for alarm signal in the plug to come forward to the dash. In looking at the schematics that do show the alarm, it feeds off the Purple and Tan /Black wire which correspond to pins 5 and 4 respectively. And that seems to be the case for all the schematics with an alarm.
I have attached my downloaded version of your engine’s schematic as a pdf file. The note at the top is from that OP who provided it.
So back to your issue: did your alarm stop working or did it never have one? If it never had one, I suspect you can add one across pins 4 and 5 but suggest waiting for others more familiar to chime in prior to messing with it.
If it stopped working, I have no idea other than its simply failed and needs to be replaced once found.
Good luck and keep posting your findings and questions.