I am not sure there is any convention for out door wire color coding, sounds like 4 conductor bell wire was used, but whatever.

Each speaker should have two wires hooked to it, if you can take one speaker down and look then the answer should be easy, if not then you will have to find out which two are going to which speaker.

If I were guessing I would try red/black & white/green to start with. I also assume you mean and 3.5mm Output not input unless you plan to attempt to use the outdoor speaks as microphones...

The prior owner likely had the speakers wired to a second set of speaker terminals on an amp or receiver, thus the lack of a common return wire, but that is a guess.

What you need to do is isolate the pair that power each speaker, we can deal with the common at the end. There are a number of ways to do this, go to Radio Shack and get a 3.5mm jack with no wires on it and so you can get to the solder connection points inside the connector shell, plus it into an output and try different combinations until you get sound out of one speaker, then mark them as needed to know which is which L/R, then the other pair should power the other speaker.

Given that they are outside and phase is likely not a big issue for you I don't think you need to figure with is the +/- or feed/return, but if one is wired one way and the other is reversed you could lose any stereo effect or at least shift it, of course if that did happen and it bothered you just take one pair and reverse them.

Providing there is no inter-speaker wiring and the 4th wire is hooked to nothing, another approach is to take an ohm meter or DVM set on the Ohm setting an you should get a near 0 or 0 ohm reading from a pair hooked to a given speaker, all other combinations will give you an open or infinate reading.

Once you know what pair of wires goes to what speaker, given that your 3.5mm juck has only got three connections on it, lets say the left speaker was on Red/Black and the Right speaker was on White/Green.

You would hook say the red wire to the tip connection, the white with to the center connection, and then twist the green & black together and hook them to the base (bottom section) of the jack. That one is usually easy to tell if there is not a diagram with it as it will be longer than the other and have a couple of nibs used to secure the ground wire.

Then you are set to go.

If the 4th wire is just in there but not used and there is a common wire between the two speakers the process is pretty much the same, some combination of two wires will put sound out of one speaker and the other combination will put sound out of the other, the common wire in this case will be the one that is needed with one of the other two wires to make sound come out of either.

Ex.

R-----|\
Speaker 1
B-----|/

W-----|\
Speaker 2
G-----|/

3.5mm Jack
R-------------------------\Tip
W---------------\Center
BG------\Common

Of course change the colors you use in the above example to match what you find with your testing, the idea is to get the tip of the 3.5mm jack wired to one speaker, the center of the jack wired to the otherm and the common wires to each speaker hooked to the base of the 3.5mm jack.