I can say with confidence that mine was not a case of the glow plug light going on and off quickly. I stared at the dash while turning the key and it did not come on. And it was a chilly morning. And there was no cranking or solenoid click. Removing & cleaning the ecu relay fixed it. Others have swapped or replaced the relay with the same result. What seems to be happening is the whole start sequence is not initiated, maybe because the ecu is not happy with the voltage (if any) being supplied by the relay at the moment the key is turned on. And like other corrosion or funky connection problems, the problem can be very intermittent and will drive you crazy.
If you park outside like I do, and don't drive a lot during cold weather, it may be that condensation in the fuse/relay box is the cause. The relay box is mounted on the firewall, which may be colder than the rest of the interior, and maybe prone to a bit of condensation in cold weather. Here is the thread that originally set me onto the possible condensation issue.
http://www.motorhomefacts.com/forum-printtopic-1-122907-0-0-asc-viewresult-1.html
Also, here is a list I compiled of all the relays with the MB and Hella part numbers. (Hella appears to be the OEM for most or all of the relays). Several of the relays are interchangeable. This list is what I found in my 2011. It appears to match the list for 2007 that was posted elsewhere on this forum, although some relays have been superseded with new numbers. But be sure to check the numbers on your actual relays; don't rely on me, just some guy on the internet, to get this right. If you order online, you may need to try either the MB or the Hella numbers to find the right relays. The "terminal" numbers refer to the European standard designations for the accessory, ignition, and start circuits.
Relays by position #.
1 horn
2 Wiper speed 1 or 2
3 Fuel pump (appears to change location in 2015)
4 Wiper on/off
5 Terminal 50 starter solenoid
6 Terminal 15R "radio" (accessory)
7 Engine control
8 Terminal 15 ignition
Replacements
1,3 MB 0045452905 (0035420219 orig, superseded), Hella 4RD965 453-04
2,4,5 MB 0025427419 (0025421419 orig, superseded) Hella 4RD 007 794-21
Originals were 20A, new are 30A
6,7,8 MB 0025422619 Hella 4RA 007 793-031
I found it particularly interesting that the starter solenoid relay was one that was upgraded to 30A. This is probably related to the blown starter fuse issue that showed up in 2010-2012 models. If you have a 20 amp relay in this position in a 2010 or later van, might be prudent to upgrade it.