I got an 8spd Alfine Di2 groupset on eBay and it seems to have everything I need. I set it up like this:
ST-S705 shifter → SC-S705 display/junction box → SM-JC41 (I think) internal junction box → SM-BTR2 seat post battery & Alfine Di2 hub (not sure what part number exactly)
But when everything is connected (and I've ensured there's a solid click when I push these cables into each component) it's completely unresponsive, nothing lights up in the SC-S705 display and it doesn't shift. It could just be a dead battery and I asked to borrow one of these to try out this theory but I'm worried if the problem is elsewhere it'll have been a waste of everyone's time.
Any suggestions as to next steps before I just take it to a shop with all the right equipment so I can pay them to point the finger at the culprit? Also suggestions for a Di2-enabled shop? Thanks!