Yeah, I wasn't sure how to explain that any better and can't picture an ASCII diagram. Attached is a rough diagram of the way my 2 HUs are wired up (I'm sure I missed a couple of the wires in there, but you get the idea).

All the connections in there are wiring harnesses so that if I want to sell the car some day, I can unplug them all. Re-connecting #1 and #2 puts the system back to the way it originally was.

So what I'd like to do is unplug #2 which exposes a forward and reverse pair of Nissan wiring harnesses and put the device in between there. If they don't sell those, then I'd go for something like:
#2 HU side ---> New Rev Nissan Harness==solder==New ISO Harness ---> "OEM-1" ---> #2 Amp side.