My 81 300DT just burnt thru 2 glow plugs of the 5 I replaced last winter. Because of time constraints, I used 2 of the old plugs (still good) and things were ok for a few days. Now my relay will not kick in and I get no light on the dash. The bulb is good and all the tests for the preglow system are positive. I read where the relay senses the the voltage(or is it resistance) between the #1 plug and the rest and if there is a mismatch, it may not work. Does this sound like what is happening or should I pony up and replace the relay and the plugs. The part shop sells a bosch relay that seems to be cheap by comparison.