Regarding the relay, if you are getting 11 volts to the glow plugs then it is working. I would put a milti-meter on one of the glow plugs and turn the key to the glow position. You should be getting about 11 volts for about 30-40 seconds and then you should hear the relay click off, and the voltage will drop to 0. Also, do not assume your glow plugs are good because you changed them 4 months ago. Unplug the connector from the relay to the plugs and measure the resistance to each one. They should be in the .5 to 1.0 ohm range. If one (or more) reads infinity it should be replaced. Glow plugs tend to fail randomly (like light bulbs).