1995 GMC C2500 lights flashing draining battery

I bought the van about a week ago and I think it has a aftermarket alarm system that doesn't work anyway when I start the fan the locks lock and unlock constantly and when they do the lights flash like hazzard lights when I kill the vehicle about five minutes later the hazzard lights start blinking and you have to start it to stop it I just keep disconnecting the battery but that is getting very annoying is there anything I can do to stop this from happening plz help me