How can I wire up a new pin on my 1999 V70's ECU?

I've got most of the clutter out on the right side of the engine compartment
now, and I'm down to the plastic base to which the ECU is attached.
I've unbolted it from the engine compartment, but it's still pretty much held down, apparently by the wiring harness itself.
underneath the assembly, however, is solid plastic.





So apparently I need to somehow release that white plastic cradle to get access to the wires from underneath?
I've searched but haven't found any instructions on how to do this.
I've done it at the junk yard, but destroyed the cradle in the process.
I'd rather not destroy it on my car!

Why am I doing this?
In preparation for my auto-to-manual swap, I'm trying to wire in the clutch sensor to the B15 pin on my ECU.
I've found where it is, but so far, I can only access it from above.
I need to insert a spare plug into what should be a blank hole from underneath.
Looks pretty impossible without breaking stuff.

Any help greatly appreciated - thank you!

 

I've taken one apart before, but admittedly, it was aluminum. My car is a 98. The geometry looks the same, though. It looks like it was just snapped in. Do you not think you can force all those snaps to release and pull it up? The one I did, everything came apart.

 

Thanks for the reply, firebirdparts!
Yeah, I may have to try to do that CAREFULLY this time!
(I trashed the one I tried to take apart at the JY)
Thanks again,