This is going to sound too simple, but I went thru a similar situation. Let me just say, check the 2 fuses that control the DME/start circuit. I think one is a 20amp and the other a 7 or so. It will say on your glove box fuse list. Give it a shot.

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600