Is the Diode burnt? If it isn't you should be able to remove it to find the correct one and when you remove it note which side the stripe is on and replace with one that is comparable. Radio Shack should have it, if they don't you will probably need to call these guys ---> http://www.diodes.com/
. They should have it for sure. We use them where I work to get all of our electrical stuff.
When you say it is wired in parrallel with the power and ground what do you mean. It should just be in line in the ground side (most likely) or the positive side. If it goes across the positive and negative at the plug or wherever then it is wired in series. This would prevent the field from regenerating current and feeding back through the positive and blowing a fuse or circuit breaker.