use an easy out extractor if the vise grips fail. yu drill a care full hole in the centre of the broken bolt and the reverse threaded extractor will remove the broken bolt no problem. be care ful to only use a tapwrench on the extractor. be very careful not to break the extractor in the broken bolt cause you are in BIG TROUBLE if you do. only EDM can get it out. extractors are cheap at your auto store. $5-10
email me if you need more specific details.
good luck. 20 year old liquid wrench should work unless separation has occured.