IIRC, it was Peugeot that had a 5/16" square plug in one of their cars. I made one by grinding down a junk 3/8" extension to 5/16". Still sits in my toolbox 25 years later.The 3/8" extension is too big and the 1/4" is too small. I have a 5/16" square socket coming from Amazon tomorrow. I appreciate the replies.