I have a 1999 Mitsubishi Montero sport XLS the starter is not engaging when I turn the switch it just makes it buzzing sound like the stars been magnetized I bought new starter and put it on it did the same thing I bench tested the old starter it works fine what could be the problem

 

Check the battery by getting it tested and check the positive and ground connections. The main ground point to the battery is located behind the A/C compressor, you can access it from the driver side fender well pretty easily. Sometimes, these cables can get corroded under the cable insulation and you wouldn't know it. I also wouldn't rule out the battery unless you already tried replacing it to solve the issue.