Rel Storm III Subwoofer with Custom Neutrik Cable

REL has their manuals online. Go to the Sumiko website. My small Q108 uses the yellow to left +, the black to left - and the red to right + and works fine. Check the website just to be sure.
http://www.sumikoaudio.net/rel/idx_manuals.htm

I do not know if the 5-wire is covered in the manual (it is not for the B1 and another REL I owned). It sounds like this might be a cable made for HT config where someone had amps for the L/R and an amp for their center and used the REL to pick up sub-bass information from all 3 channels across two amps via 4 of the wires with the 4th being the ground. I'd suggest giving Sumiko a call to determine what the extra 2 connectors are for. What custom cable manufacturer is this?

Sumiko may have no knowledge that could help you due to the custom nature of the cable. At the very least, you could always order a replacement standard REL/Storm cable.

I had a custom cable made for my Stadium and REL was able to advise me how to connect it; call them.