If the other side isn’t set to some sort of GND, how will the motor initially start? You would have 12V on one side and the other side would be disconnected. When at one extreme or the other, one of the contact switches is open.
The way the limit switches are wired, when activated the other side of the switch gets set to GND.
You can give the motor a return path through the relay contacts. What hekk is describing is essentially an interlock circuit. The limit switch doesn’t have to provide a return path for the motor just the control signal, you can then control the motor voltage and provide a return path through a set of normally closed contacts on the relay for the opposing direction. As long as both directions aren’t activated at once there is always a return available through one set of n/c contacts or the other.
I feel dumb, but i haven’t been able to puzzle out what you guys are saying. I’m not sure what you mean by control signal, I’m thinking of relays that are either energized or de-energized.
The “power” part of the circuit is the motor supply; the “control” part is everything that goes into determining when that power will be applied. So the limit switches and start/stop, etc. are control signals. You’ll need two relays: one for CW control, one for CCW. The relay has an armature which you will turn on and off with your control signals and it has contacts which you can use for power or control.
I’ve been stuck trying to deal with a sick kid last night and again tonight so haven’t been able to draw out the circuit. A picture’s worth a thousand words easily with this stuff. You’re on the right track in your picture I think. Just needs some fine-tuning.
Any thought that’s put into this that isn’t my own is much appreciated!
Do you know anything about these limit switches? Do they have a current or voltage rating? Do the CW and CCW wires in your diagram have 12V on them like the motor does? It would be easiest to have the relay armature voltage coming from those limit switches if it’s possible.
I probably shouldn’t have labeled them CW and CCw. Those are just inputs that need to be attached to GND when the switch is closed. The only place 12 V is available is on one motor control wire or the other depending on direction.
It might be easier to do this with a logic gate. The limit switch is normally closed?
At one extreme, one switch is open and one is closed.
At the other extreme, it’s swapped.
In between, they are both closed.
Here is another attempt by me using 2 spdt normally open relays. It still uses the switch gnd as the negative pin of the relays, so I don’t know if it’s any improvement over my last design.
Didn’t draw in the switch signals going back to the black box, I think that was just confusing the issue.
Didn’t actually draw in the switch lines going to the negative pins. Would have made it messy. 
The big problem with any idea right now is whether you’ll damage anything by applying 12V to one of those limit switches. They are currently used as dry contacts by the sounds of things and might very well be able to handle switching 12V but they might not as well and the IC chip almost certainly will not accept 12V at the input where the leads are currently connected. It would be pretty straightforward if the limit switches were also switching voltage in the current set-up rather than just switching ground to the IC.
I can use the switches for GND (although that’s not ideal either), but I don’t think I can use them to switch 12V.
Here’s a cute design using a single 2-coil relay. Still uses the basic idea of grounding the relay through the switches though.
I hate to see you try it and ruin some of your equipment. You don’t happen to have a data sheet or anything that came with the device that might have some more info, particularly regarding the limit switches?
The switches just say microswitch and freeport, il. No part number that I can see, but the leads look pretty beefy.
What damage am I risking? Won’t the switch leads just have to handle the current of the relay coil, which will be on the order of afew milliamps?
The leads themselves might be OK. If you leave them connected to the IC though, it is just designed to see ground when the switch is made so that could get damaged. The other area of concern would be the contacts of the switch where it interrupts the current. There’s a lot of arcing with DC switching because you’re always interrupting the maximum voltage and there’s an inductive surge usually when the switch opens. With AC the voltage will eventually reach zero volts at some point during the switching action so it’s considered to be somewhat self-extinguishing in terms of the arc. Maybe these switches will take the abuse maybe they won’t, it’s hard to say.
Good points!
I’ve let the smoke out of a device or two over the years. 
I think the flyback diodes I drew in would help with any arcing, but I just found out that the switch isn’t connected to ground. It’s connected to a pull-down resistor. So I might be back to the drawing board.
Freewheeling diodes help minimize arcing but they also slow down shut off of the coil: link to PDF. Might not matter enough for your application but won’t hurt to be aware of it. Is the speed of your application consistent enough that a timer activated at the start of the motor cycle could be used rather than the limit switches? That would bypass the issue of the switches being connected to TTL circuits and would be consistent but won’t be any good if the speed of your application changes a lot.