Sounds like it could be metric fine, 12mm x 1.25mm. That is two thread pitches down from the normal metric coarse of 12mm x 1.75mm. This one is commonly used in automotive applications so a good bolt warehouse should be able to match it up.
For info:
12mm metric threads, M12 x 1.0mm, M12 x 1.25mm, M12 x 1.5mm and M12 x 1.75mm are all standard items.