If you're concerned with fading from front to rear as well as a dedicted sub woofer output you will need 3 RCA outputs - Front, Rear & Sub.

If you're not concerned with fading from front to rear 2 RCA outputs is adequate. More than likely if your deck has 2 preouts they will be labeled as front and rear with rear being switchable from full range frequencies to sub frequencies. In this scenario, you would run the RCAs from the HU's front outputs to a 4-channel amp that would power all 4 speakers. You will want to verify that your amp has the ability to take one input source and distribute it to all 4 channels. The sub would be running off of the HU's rear RCAs.

Another scenario would be to purchase a 4-channel amp that offers RCA preouts on it. This would allow you to attach the HU's front outputs to the amp's front inputs and the HU's rear outputs to the amp's rear inputs. Then you would attach a second amp to the first amp's preouts. This setup would allow you to control front to rear fading but eliminate any control of the sub woofer amp from the HU itself. In this scenario, the first amp's preouts are duplicating the HU's rear output. So, when you controlled front to rear fading from the HU the sub amp would be controlled by the fading of the rear signal.

I would recommend buying a HU with dedicated front, rear and sub preouts. This will give you the flexibility you will probably want in the future.