If I was to buy a factory sub unit with the amp can I hook it up to my OEM single cd deck? This would all be Ford OEM components. I want just do a small increase, I worry about theft and would rather have the OEM stuff stay there than buy a nice new aftermarket deck to get stolen. I have a 2005 XLT Scab

 

No you can't, the only way to add OE sub is with audiophile head unit and they do not make an audiophile single cd. You would have to upgrade to a six disc.

 

You could add an aftermarket sub and amp, while keeping your existing OEM headunit. You would just need to use a line level converter to convert the speaker leads to an RCA output to run the amp.