They probably ran an amp, so they used RCA cables to run the "sound" to an external amp. Are you using the HU to power the speakers, or do you have an amp? If not, find wires on factory amp under dash, splice onto new speaker wires out of new HU. Hook 12+, mem, and ground wires up to new HU as they were on old HU.
Scott

Scott Mabe I WILL be Phil. 1999 Contour SVT, Tropic Green BAT kit, K&N RU-3530+MAF, Magnecore 8.5mm, some audio goodies, Hightowers, Knauberized, tatoo by Parkedcar.