It's most likely the wiring is already connected to the timer. you just need to connect whatever valves, piping and drip lines you need for the back yard.

There is only one set of wire in the timer. And there are two separate sets of wires, one to the front yard and one to the back yard. I'm wondering if they forgot to pull it through the wall where the timer is.

you should be able to open the panel of the timer and see how many stations plus the common are wired in the clock and match the color of each wire to the proper valve. they most likely ran the wiring through one set of valves to the other.

OK, thank you, I'll check that out.