You can jump the Coin relay off of the Lock Relay (has same black neutral)
and see if it energizes.
So the Coin Relay does nothing. Not even a quick pulse?
There's a make/break switches on Score Motor 5b. I'd maybe check them
for cleaning and adjustment after making sure their switch stack screws
aren't loose..

So when you manually trip the Coin Relay, the Score Motor does energize. That's a direct feed..
This might seem too simple, but there's an Anti-Cheat switch that if dirty or not made, breaks
many of the paths to the Coin Relay, yet other Relays will continue to function.
You'd think the Anti-Cheat would trip the Game Over Relay, but it looks like there's one that
breaks the path to the 10 cent, 25 cent, Coin, ect. Relays. I have a Grand Prix. I can check tomorrow,
but my guess is that that Anti Cheat Switch would be on the inside of the Coin Door like most.

Are you sure the Coin Relay isn't energizing, but the Score Motor isn't running.
As long as the machine is on, any time you manually activate the Coin Relay,
the Score Motor should turn..
You could put a hot wire on one of the red credit button switches, and then a
wire on the other switch and feed it to the red wire on the Coin Relay, and that
should activate it..
But first, if the Score Motor doesn't turn while manually pushing in the Coin
Relay, I'd first consentrate on that..

The Game Over Latch Relay trips just a split second after the Score Motor begins its revolution.
When facing the Score Motor from the front, it's the bottom set of switches in the 2nd stack from
the left that trips the Game Over's Latch Relay at start up while the Coin Relay is energized.
The Red Credit button (I have mine on free play) energizes the Coin Relay. The 4D set of switches
(4th set from the coil) in the Coin Relay energizes the Score Motor, and the Coin Relay remains
energized pretty much right to the end of the completion of the Score Motor's revolution..
If the Coin Relay isn't energizing (has nothing to do with the Game Over Relay contacts), you
could jump a wire from a common Yellow to the Coin Relay and it 100% should energize.
If so, you can just splice that red credit button set of switches in between that jumper wire, and
you should be good to go.

Yes, there's 2 black wires on one lug, and a red on the other. The red one goes to the Coin Relay's
hold switch with another red wire. With elegator clips, I'd try jumping a yellow (common) wire to
that hold switch with 2 wires. That should energize the Coin Relay. If so, you can tie that wire in
the Red credit button set of switches, and that should do the trick as long as the Coin Relay energizes
when you manually activate it..
btw: The Coin Relay itself has a switch to jump from that has a Yellow common wire..

No, I wasn't referring to a switch in the Game Over Relay.
I was saying that there's a switch with a yellow common wire on one of the switches in the Coin
Relay itself. If you jump that yellow to the red wire on the Coin Relay's coil, the Coin Relay should energize.
If so, you can take the jumper off of the lug with the red wire, put it on one of the Red
Credit button switches, then feed another wire from the other Credit button switch to
the red wire on the Coin Relay. Now when you press the Credit button, the Coin Relay
will energize and shouldn't have any interference with any of the other functions.
I might add, this is if you only care about the game to work and play. If you want it 100% factory,
you'll need to find the bad connection from the Credit Unit's switch. Maybe the Jones Plug
carrying the feed.