center channel speaker

If you use the sub preouts on the head unit (and keep the low pass filter turned off), you can get a mono signal from just one rca plug.  So all you gotta do is take one RCA plug from a standard cable, or go to radioshack and get a single plug cable, and connect it to tone of the head unit's two sub rca plugs (doesn't matter which one you choose).  Then plug the other end of this cable into your center channel speaker and you're all set!

Since it's a mono output for the subs, both plugs are sending out the exact same signal/sound, so omitting one of them will not cause it to sound any different.

Just FYI, If it were not a mono output (if you used the front RCA preouts, for example), then if you combined the two separate plugs you'd probably get distorted sound coming out of a single output plug.  More info on that here.

when the filter is on, the signal output is still mono, but it's limited to the low frequencies only (varies on what cutoff point you choose).  As long as you leave the filter off, you should get a full frequenc, mono signal coming out of each of the sub plugs.  The signal is adjustable via the head unit, regardless of whether or not the filter is on.  So you'll have to control the volume of the center channel using the head unit's subwoofer level control.

You could also use the front outputs and a Y-cable, but I'd put some resistors in there especially if the front preamp outs are being used by something else....a resistor from the L+ to the C+, and a resistor from R+ also to C+. And all the -s would be common or if for some reason they are isolated  (doubt it) you could do the resistor thing on the grounds as well.