Have in construction a towbar (legal on pre aug 98) but the two Threaded bolt holes at the front of the trunk floor are defeating me On a convertible these would be where the braces attach behind the differential. I have tried M12 1.75/1.5/1.25 but no go . Perhaps I need a tap to clean out the threads but which one? US towbar maker CURT says M12/1.75 but it does not even bite.