I was going to put it up against the firewall like on the MKIV, but changed my mind after this thread...
http://www.supraforums.com/forum/showthread.php?t=514936
This was my contribution, after mounting the ECU in the same place as the 7M's...
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Sorry for the quality...it was at night and the glovebox was already on, so the angles aren't what I'd normally provide...
As you might be able to tell, the ECU squeezes up REAL tight to the ABS ECU and between the space above the A/C evaporator housingand the headlight retractor module mounted on that reinforcement bar.
You have to pull the bracket towards you (the one that holds the latch hook), then try to push it back, once the ECU is in. You may want to hammer it upwards a bit more than what your pliers may allow. I did not and then had to bend the hook further after assembling everything.
All brackets of the ECU must be removed. Once you install like this, it will NOT move.