E30 Zone

Is there any good reason why you couldn't use Motronic Pin 27 (start input) as the +12v source to power Megasquirt?

Besides the fact the cable looks a little skinny, its probably the most simple method of making the thing turn on and off with the key.

You may be able to use it to directly power the MS, but best to ensure the supply has a fuse of suitable rating, or better still, use it instead to power a relay-switched feed to the MS ECU.

Last MS I did I ended up putting a direct feed onto a switch, was very handy when tuning/fault hunting and burning firm ware. However that was not a normal road car so a switch on the MS feed was handy as a kill switch/fuel shut off switch.

On the E30 you can take a feed off the ignition switch directly with no problems.
Pin 27 is actually just a feed from the ignition switch so it would not actually be hard to trace it back and upgrade the wire to a larger gauge, adding a fuse or a cut off switch at the same time.

This is how I wired mine

http://farm4.static.flickr.com/3096/259 ... 6bfc_o.png