I am assuming this is the WINEGARD RPGM52 Power Cord you are referring too. What part of it fails.. This is just a 50-foot extension cord for 12VDC... AMAZON has it listed for around $28

Google Image
If the two pin connector is getting corrosion than maybe you can rig up a 1-inch piece of heat shrink tubing to go over both the connections to make it more weather proof... Hardware stores like True Value - Lowes sell short pieces of heat shrink material in a 4-6 each to a package.. You can tie wrap or heat shrink one end and allow the other end to slip over the receptacle perhaps...
Roy Ken