Hey OP I had the same problem once and fought it for an entire season. Finally having had enough frustration I went with the plug/cord set like posted by Gdetrailer. I replace the plug a few years ago and have not had a problem since. If and when the plug gets loose I'll spend another 20 - 30 dollars and replace it all again.
Gdetrailer wrote:
For less than $30 you can buy not only the plug but the entire harness with the plug molded in to the harness with 8ft of wire..
FOUND HERE
That one looked interesting since it has double prong contacts in the plug rather than standard single prong contacts..