If pressing the Credit button doesn't activate the Credit relay,
Inspect and diagnose this circuit with Alligator clip jumper wires

https://pinside.com/pinball/forum/topic/gottlieb-big-shot-repair#post-6305153
http://www.planetimming.com/Pinball/troubleshooting/EM%20Troubleshooting.pdf
http://www.pinrepair.com/em/index3.htm#features
Example of a pinsider actually doing this https://pinside.com/pinball/forum/topic/red-baron-tech-question#post-5858156

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There are 5 switches in the circuit I posted. Do you see them? Any one (or less likely more than one) could be at fault.

The simplest (but not 100% reliable) way to find out which switch is at fault is to jumper over them one at a time and test.

In case there's more than one switch (or solder joint etc) at fault, it's more thorough to jumper over them in combinations, starting at the Yellow wire on the transformer.

For a switch to work 3 things are necessary:
1) When open, there should be a small space between the contact points (duh)
2) When closing, the long blade's contact point should push the short blade's contact point enough to move the short blade
3) The contact points should be clean, which they usually will be if #2 is happening

You didn't say which wires you jumped from and to.

Let's start by guessing that maybe the problem is to the left of the Credit button on the schematic, namely in the Reset relay switch or the Motor switch labelled IND-A. Do you see those switches on the schematic I posted?
The next step is to test our theory. We do that by running a jumper from the yellow wire on the Reset relay switch that has a Blue-Yellow wire on its other terminal to the White-Gray wire on the Credit button. Do you see how that eliminates the Reset relay switch and the Motor switch from the circuit?
Now press the Credit button. If the Credit button now works, then our theory is correct and we've narrowed down the problem to the part of the circuit left of the credit button.
If the Credit button now still doesn't work, what does that mean in terms of where the problem might be?

If the Right Eject coil doesn't activate, there are several possible causes, but the first one to look at is
Inspect and diagnose this circuit with Alligator clip jumper wires

https://pinside.com/pinball/forum/topic/gottlieb-big-shot-repair#post-6305153
http://www.planetimming.com/Pinball/troubleshooting/EM%20Troubleshooting.pdf
http://www.pinrepair.com/em/index3.htm#features
Example of a pinsider actually doing this https://pinside.com/pinball/forum/topic/red-baron-tech-question#post-5858156

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Yes, check the Aux R. Eject relay switch: For a switch to work 3 things are necessary:

1) When open, there should be a small space between the contact points (duh)
2) When closing, the long blade's contact point should push the short blade's contact point enough to move the short blade
3) The contact points should be clean, which they usually will be if #2 is happening

Check wire colors to find the switch on the Auxiliary Right Eject relay.

I don't know. Check the wire colors.

From https://www.ipdb.org/files/1550/Sonic_1977_Mars_Trek_Schematic_Diagram_continuous.pdf
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