For the last few months I've been running one of my ER9 bikes (1 of 2) with an externally routed RD battery cable. Today it bugged me enough to try and route the cable through the chain stay. Months ago, when I initially installed the groupset, there was any obstruction that would not let me put the cable through the RD chain stay. It appeared to be some sort of residue blockage.
I first tried forcing the battery cable through, but it ended up getting jammed and I had to pry it out. Unfortunately by forcing the cable out, I bent one of the gold pins on the cable head. I thought for sure I messed it up and would have to order a replacement. I took a minute to think and decided to try and fix the pin using tweezers and bend it back into place. Thankfully that worked and the pin was fixed. I still had to deal with the residue obstruction. I then thought about heating up a small screw driver and melting the residue. Luckily that tricked worked and I was able to get the battery cable through and properly route everything internally.
Seriously, working on these cheap bike parts is almost like a form of self-punishment!