You mean you applied AC Accessory voltage directly to the signal head 4-pin? That would in all likelihood blow up the internal LEDs. In which case you'd have to see if MTH sells replacement signal heads (which you'd think they would since apparently the stock bridge has locations for 4 heads but only supplies 2). If attempting to repair a head, I'd think it comes down to whether you can peel/pry/pop open the head to expose the LEDs. Since LEDs should last a lifetime, I can't imagine it was designed to allow easy replacement of individual LEDs. But if you can open it up and take a photo, I'd think the actual LED type would be easy to find for a nickel a piece or so.
As to working with what you think are the known-good heads. What I'd do is test it without the 153IR. As the instructions say, apply AC common to Black. Then apply AC power to Green. The signal head plugged into the corresponding bridge socket will turn diagonal, then vertical after a few seconds. Then remove the AC power to Green and apply AC power to Red. The Horizontal aspect should come on until you remove AC power from Red. If you can't get this behavior then it won't get any better using the 153IR!
