Macioslaw

For the longest time I have been trying to hook up this piggy back that I got from a friend a while back. The issue is though when I do hook it up the car starts up fine and runs, but when I put it into gear after about 15 mph the trans locks into 3rd gear and D5 starts flashing....any ideas what it can be, Boomslang harness? I have a 03 TLS auto