2000 Focus w/ 116000 mi. Battery light came on but car starting & running fine. Took it to shop thinking it was alternator. Alternator replaced and now instrument display has gone wild w/gauges reading normally, then wildly, then not at all. Thought it might be instrument cluster. Checked plug and it was tight. Next time started getting surge of power to windshield wipers and interior ventilation fan. Final symptom is crackling sound in radio speakers when radio not on.
Perhaps someone else can verify this, but I’ve been told that the early Ford Focus had more Technical Service Bulletins (TSBs) than any other car, ever. The previous winner had been the '84 Corvette. Does the shop you are using have access to TSBs? Not all do. You need one that does.
I believe it was recalls more than TSBs that hurt the Focus early on, but I’m seeing a ton of TSBs as well - of course, from my experience, TSBs aren’t always an indicator of a problem as they often are for an improved diagnostic or repair technique (for example, there are 4 TSBs for the 2000 Focus with the exact same fuel gauge issue, each one successively updating the method of diagnosis and repair to make it faster and easier)
Still, an excellent suggestion on where to start.
I’d look at these:
TSB #18581 – ERRATIC INSTRUMENT CLUSTER GAUGES AND/OR WARNING INDICATORS - SERVICE TIP. *TT (NHTSA ID #10017884, APRIL 01 2005)
TSB #15317 – SOME VEHICLES MAY EXHIBIT GAUGES THAT SWEEP CONDITION. *TT (NHTSA ID #630229, OCTOBER 01 2001)
TSB #7429 – SOME VEHICLES MAY EXHIBIT GAUGES THAT SWEEP ESPECIALLY AFTER STARTING THE ENGINE. *TT (NHTSA ID #610816, NOVEMBER 01 1999)
Still, I wouldn’t buy a 2000-2002 Focus. 2003 or later, sure… they had low problem rates then. But not so much for the early ones.