The OP's mustang uses the J1850 PWM protocol for that a simple serial cable will not work. A simple serial cable can be made to work for the ISO protocol found in most EU cars but if you have a vehicle with a non iso protocol you need a protocol converter.
OP check out our interfaces at
www.obdpros.com, they also work well with PCMSCAN software.
Thanks
Paul
www.obdpros.com