#4 fuse keeps blowing
 

I have a 115 chassis 1976 300D. I recently had the starter rebuilt and converted the glow plug system to the pencil type with a manual relay switch. Now each time I turn the key to the #2 position, fuse #4 in the main fusebox blows. This apparently cuts off the power to the 12 volt keyed source, and therefore the glowplugs will not light. Does anybody know how I should begin troubleshooting this problem, or a likely cause for this. Any input is appreciated!
Thanks, Marc