Car no start melted wires

So my bro just recently installed a new starter on my car 4 days ago
and when it was finished it started right up i drove it for 2 days no problem at all.
so i washed my car yesterday start it up n drove around the i turn off the car for a sec n turn it back on to go into the garage it wouldnt start i start smelling smoke n seeing smoke and i see my negative wire for the battery melted the plastic around it,and also the postive for the starter melted too and melted the black cover around it i think its part of the harness wire tht also goes to postive terminal on battery…but i see a loose peice of metal plate near my starter tht looks like it suppose to be screwed in somewhere im thinking its a short,and my damn brother didnt screw it in where it suppose to bhe…HELP ME OUt is that the problem??