Transmission service

I have a gallant that at first would not go into reverse. After awhile the whole thing wont go into gear I found out the person who changed the tranny fluid didn't change the filter; DAH? I am certain this may be a big problem but I also have a electric plug to the top of the tranny that has no electric to it. I tested it with a test light and got nothing. When starting engine and putting into gear it jumps but never goes into gear forward or reverse. I hope putting new fluid and filter will help but this plug has me bothered and I need to know is there a possibility of worn wire that would stop from changing gears or clutch. I know fluid has to have new for pressure sake. But the plug is gray and 5 tongs on one side. Is on top under battery. Thank you for your help.

Feels like transmission is shuddering @45mph. Or riding on an old rocky road. Only happens around 45 to 50 mph. Changed transmission fluid, can not find filter. Any ideas on what it could be or what I can do to fix this problem?