Well, lets assume that the way it gets worse is that the Cubans and thier allies among the Grenadians, forsaw the invasion and set up a cell network of terrorists and they begin a campaing of bombings and assasinations after the fighting dies down.
Results?
Minor victory for Reagan becomes minor embarassment.
More aggressive policy Reagan advances takes a hit, perhaps leading to greater caution later.
If bad enough, maybe no Iran-Contra scandle and/or more resistance to high tech missiles being given to the anti-soviet forces in Afganistan.
No iran-contra leads avoids a scandle for Reagan, might help G.H.Walker get elected, maybe even reelected.
No stingers for Afganistan? Less effective anit-aircraft leads to bloodier, longer war, although probably not Soviet victory since other systems would still be sent.
As a sole POD, probably not enough to change outcome of US elections or Cold War.