Just a FYI. The 1990 Ford Mustang O2 sensor is a three wire Bosch unit. It is the same as the three wire Bosch O2 sensor used on W124's and W201's, the exact same sensor! Cut the plug off your old sensor and splice it on the new one. The OE Mercedes sensor is around $175 and the Mustang O2 sensor is around $36.
I had no problems passing emissions once my O2 sensor was replaced.