[Decolliber]

Car in question is my daughter's 1993 Ford Escort with 110K miles. Used for high school & college transportation by two kids, and badly treated by both. Dented body work, rust in wheel arches, but still basically functional. During the past two years I have spent a lot on it: alternator, brakes, battery, ignition switch, timing belt. The latest thing is broken rear trailing arm, spindle and axle ($580). Considering what I have already spent, I decided to go ahead and have it fixed. Every time something goes kaput, I think "Well, I have already spent $x in the past year, I should spend a bit more and it should be reliable for another few years".... and then a few months later, something else goes bad(*). But at some point I am going to have to make a decison to junk it.
Any good rules of thumb, or more complicated algorithms with which to make a decision?

(*) yes, I am aware of the fallacy of considering sunk cost instead of marginal cost.