O/d Off Light On Manual Transmission 98 Gt. Normal?

Sorry for the vague title. I have a 1998 GT convertible manual transmission and I ran the VIN code on Ford's Fleet site to verify it was from the factory a manual, someone else said maybe it was converted. The O/D Off light is constantly on the dashboard but I thought that was an auto transmission issue. I was just wondering if anyone could think of why that light might be on. I searched but it brought up a bunch of topics relating to auto's so I'm sorry if it's been answered before. TIA for any help!

It seems so random that someone would swap the ECU like that but I suppose it could have been. Someone else had mentioned maybe a shorted wire somewhere. I just wanted to make sure that nothing was going to be hurt by the light being on. Thanks for all of your guys advice