OK - I'm "building" BEC, so I'm trying to prepare for unexpected problems
My Roadster will have electric reverse with VAG starter motor. It's because VAG starters turn counterclockwise... You can use most of them, but they have different sizes.
The gear you need is 12DP - the bigger you fit the easier/slower will go
Very helpful pictures:
http://www.locostbuilders.co.uk/photos.php?action=gal&user=40inches&folder=Electri c%20reverse
As for electric diagram... I plan to connect relay(NO) feed to neutral switch, and second relay(NC) feed to brake light switch - in this way I can reverse by brake pedal.
But this is only my theory