the previous link is the way to go but make sure you seal the grommit with silicone as i found out that if the drain in the bulkhead becomes blocked with leaves then water will come in at this point especially when taking right turns and will trickle nicley down your ecu!!!!.
The day I bought my first 155, I put it back on the drive, popped the bonnet, looked puzzled and rang the dealer asking the best way to do this. Then I took pics (later) and shared this info! If anyone else does anything similar, then please take pics and send it to me for the site.