Or any car using two-wire power door lock motors.

You can easily retrofit foxbody door lock motors to your early Mustang. Below is the wiring diagram to hook up one of those inexpensive keyless entry systems (around $20) available on ebay or amazon




Depending on your exact lock motor, you may have to reverse the two wires at the lock motor, ( if it locks when you push UNLOCK, for example) but this diagram will replace the vague diagrams that come with the keyless entry kit.