I recently acquired a Mars Trek (2nd pin, first EM) and it has an odd issue. When I hit start, the "3-player" spot on the backglass lights up, and pulsates. After a few seconds it switches to "2-player", which pulsates. After a few seconds it switches to "1-player", pulsating, then solidly lit. At this point I can mostly play a game (scoring might be a little wonky, but 1 issue at a time here). I can hit start again to switch to "2-player" (no pulsating, just solid), or 3 player. I can mostly play a 3-player game with no issue.

If I hit start once, and the "3-player" lights up, and I hit start again, it will go straight to "4-player", solidly lit. Typically, if I go that route, it will play through players 1-3 normally, then it will give infinite ball 1's to player 4.

After combing through the machine, I see 2 wires that are disconnected. The first is in the backbox - some yellow wires that should be attached to a switch on a scoring reel, which came loose. I'll be resoldering those tonight. The 2nd is a grey wire (looks aftermarket) attached to the switches on the white plastic wheel inside the cabinet. I don't really know where this should go. Also not sure if it's related to my current issues.

Obviously I'm still learning, but any help would be appreciated

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You'll have to excuse my ignorance - what is a wire tracer?

I will try to figure out how to read my schematic, but it may take a while. Thanks!

Ah. No, unfortunately not. Almost all of the wires in this pin have fabric sheathing. This one doesn't. From what I can tell, it just runs back and forth across the top of the switches on the score motor.

I'm reading through the stickied posts here, maybe I'll be able to figure out the schematics at some point.

I'm trying to wrap my head around what would cause the game to slowly drift from a 3-player game to a 1-player game.

I haven't. I will take a look tonight! I'm still reading through the "basic to not-so-basic" sticky on this forum. It's fascinating.

Took a look at my coin unit. It has clearly broken in half and been glued back together. I don't really see why that would cause any issues, though. All the arms seem to be in contact with their respective points. Any thoughts?

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Some of the arms seemed to be "between" contacts. I bent those a bit to make better contact, and gave a very light sanding to the contacts with some fine grit sandpaper.