So let me see if I have this straight?...
In the "On" (I) position, everything is normal. The dash comes on and goes through its checks, headlights work, etc?
In the "Start" (II) position, everything goes dead?
If that is the case then you have several possibilities (in most common order)...
- Bad battery (you already tried another).
- Loose battery cables (should be tight if you tried another battery).
- Loose ground wire. There are 4 places to check...
- Battery to engine on left side of engine.
- Engine to frame. One on left front of engine, the other on left side of frame.
- Big cluster of grounds under the rubber flap under the driver seat (or left rear seat if an X4). Make sure this is not corroded and is good and tight.
- Loose positive cables on the solenoid or main fuses in the battery box. Check them all.
- Loose main cable inside the fuse box.
- Bad ignition switch.