Yeah, must have a good earth. I get called out to so many cars that end up being bad earths and/or low battery voltage, it sends most cars ecu's haywire. (Or like this post, having issues with hicas and finding its already been deleted!!! LOL)
Also, relays get forgotten about a lot. At the end of the day, they are just a switch with a coil winding inside, they are bound to give up the ghost at some stage.