Does it look like this?

You'll have to leave it switched on if its locked, as in leave it switched on, don't turn off the igniton..Until the dashes reappear.

Some Ford radios will give you another chance to enter code if you press and hold 5 or 6 whilst powering on.

Like you I worked on Fords back then (and still do) but struggling to remember what button enters the code. On the more modern units you enter the 4 digits using presets 1-4 then 5 to enter.. have you tried that?

Quick Google suggest on Ford 3 digit codes, preset no 4 enters it.

http://www.davidsonelectronics.co.uk/code_entry.htm