OK, you have a few options. If the deck has a front and rear outputs, I'd run that to my four channel amp. Now, the amp should, hopefully, have a single output that you can run to your sub amp. This option, if available, should sound much better than the current setup.

Now, the truly correct way would be to buy a new deck with 3 pairs(six) of RCA preouts. Front, Rear, Subwoofer. This way you have full control over fade, balance, and subwoofer level(granted the deck has that feature).