I picked up a 496 Mag HO less the harness and key switch so ordered the correct part numbers per Merc website and plugged them in and went to roll the engine over on the stand. Turning the key to run I get the fuel pump going and then nada when I turn it to start. Hooking up a remote starter switch to the starter it fires right up, so it's something I'm doing wrong, not an unusual occurance...

In looking at the wiring diagrams the red/purple off the key switch is mated to an orange wire coming from the harness of which mine has none. Also at that connection is shown a red/purple wire and I have a 10 ga one in the harness, so I connected that to the red/purple from the key switch.

The yellow/red wire from the switch is attached directly to the yellow/red wire from the harness. There is no throttle or shift so no neutral safety switch in the line, should not matter is what I discern. The purple wire from the key switch is attached to the back of the tach battery lug.

So the starter isn't getting the start signal, appreciate your help.

Red/Purple to 20 amp inline fuse to key main
"I" key terminal Purple to kill switch then to gauges and back to engine connector
"S" key terminal Yellow/Red to neural safety to slave solenoid or slave relay

Do you have the 10 or 14 pin connector?

[ATTACH=CONFIG]566723[/ATTACH]

10 pin.
Plugged the main harness into the bulkhead at the back of the engine then ran the cable to a temp dash with analog gauges and the key switch. Connected the purple wire to the batt post of the tach per the drawing, red/purple off the harness, a thick 10 ga to the 16 ga red/purple of the key switch and the yellow red of the harness to the red yellow of the key switch.
Looking at the drawing you attached, it seems all there- the yellow/red is at the inner post at the back of the starter, as delivered and I have to assume it terminates at the starter slave, it's all buried in the factory loom on the engine.

There are two Yellow/Red wires, one is 12AWG and the other is 16AWG.
The 16AWG wire comes from the key start position and connects to the small terminal on the slave solenoid. This wire is used to energize the solenoid, and the other small terminal on the solenoid goes to ground.
The 12AWG comes from the starter solenoid (top of starter) up to the large terminal on the slave solenoid. This wire connects to the Red/Purple wire (comes from 50amp breaker) when the key energizes the solenoid.

So make sure your getting 12V to the small Yellow/Red wire (small terminal on slave solenoid) when you turn the key to start position.

Thanks, I'll go look at that. I should have mentioned initially that I get a click when I turn the key to start, from the plate area of the ECM, I'm going to hope that's the location of the slave solenoid.

Sorry, a click from the PCU board.

That's where it is, and those solenoids go out every so often

Edit: same location, and may be a square. If you don't have one of the typical solenoids, then you will have three square relays and one of them is the start relay

[ATTACH=CONFIG]566724[/ATTACH]