Since VDO doesn't make any 5-inch in-dash tachs (that I could find), I'm thinking of this one from Autometer:
http://store.summitracing.com/partdetai ... toview=sku
Has anyone run this particular tach, or any Autometer tach, with the IDM? My harness doesn't even have a CTO connection.
EDIT: Did a little better search and found this nugget:
On my EDIS 4 Pin 11 or pin 2 (CTO) will not drive my Autometer tach.