Late reply but if the three spare wires that go to the unit need constant, acc and earth couldn't you just wire them up that way and still have the other features work?
Hey mate, i've got a ARC sidemount for sale.
I bought it site unseen for my 180sx only to find out later that it's for a R33/Stagea.
My loss is your gain, $200 plus delivery from Sydney if you want it.