ok i actually have a 99 mailbu and my cap also just came off, my husband had to litterly pry it off. that's the only way that thing comes off.
I don't have an answer I am also wondering since I went to take mine off today and the inside apparently that has the threads connecting it to the overflow tank stayed on connected and the outer part that you twist came off so now I am stuck with the plug still in and no way to get it out.
Dawn
I had the same problem with my 2001 Malibu. The cap is broken and needs to be replaced. not expensive at all.
Brett