the cubs had already lost one of their announcers, harry caray, prior to the 1998 season when jack brickhouse fell ill. he had retired after the 1981 season, but was still around the cubs quite a bit. his trademark call - 'hey hey' - was and still is a big part of the cubs' lexicon.
in august of 1998, brickhouse died of cardiac arrest, and the cubs added a 'hey hey' patch to their left sleeves (the harry caray memorial patch was on their right) for the remainder of the season. there aren't a lot of cards showing the patch as it was late in the season when it was added to the uniforms, but you can see part of it on this 1998 pacific home run history card
it's there above the cub logo. this is what it looks like.
it's there again on mark grace's 1999 fleer sports illustrated card
the best view of the patch, although not entirely accurate, comes via this 1999 topps gallery kerry wood card