Many of the cases that are available for your iPhone were not designed with cables in mind.   The opening provided around the iPhone's 30 pin connector is just too small to attach many accessories.  

If you are seeking a quality case that provides enough room to attach your Redpark cable to your iPhone...  check out the iPhone cases made by iFrogz.   They protect your iPhone while allowing plenty of room to connect cables.  

http://ifrogz.com/product/IP4GLL#overview

http://ifrogz.com/product/IP4GLLPS#overview