This is a drawing I did for someone else here some time ago. Take a look at it and see if if you understand it.
The primary difference is the drawing uses two low level (+5 V) signals, UP/DOWN, but not both to activate. Basically what you want to do. Where the drawing is labeled control, it basically means a contact closure to ground. So, one or the other of the +5 volt paths would be engaged at a time. You could change that to 12.
The issue is that one relay will be on at each limit.
The limit switches do not have to pass the motor current which is a good thing.
The diode must be rated for 20 Amps or so.
The bad part is that I used DPDT relays which you can make with two of the Bosch relays in parallel or get a DPDT version.
Changing the momentary signals to steady signals can be done with a dual coil SPDT magnetic latching relay such as this one:
http://search.digikey.com/us/en/products/DS1E-ML2-DC12V/255-1475-ND/465958
With a magnetic latching relay with two coils, if power is steady applied or momentary applied, the state sticks.
For now, take my word for it.
I think the circuit will oscillate if you push both at the same time. Not a big deal.
Not sure what can be done to reduce the power consumption when the system is at the ends.
See if you can understand the posted circuit and whether you can live with one relay always on. Not sure I can fix that problem.