I'm installing a sub in the stock location and would like to use the rear left speaker as a high level input to my amp. I figured overall sound quality would be better if I disconnected both rear speakers instad of leaving just one connected.
Is there any chance of damaging the stock head unit operating with only one rear speaker output connected. It seems to me that manufacturers used to advise against this practice, but it has been a while since I've messed with stereo wiring.

 

If you are looking for a combined mono signal to go into your mono sub amp, then you will want a configuration that will incorporate the use of both channels. If you only pull the signal from one side, you could be missing low sounds from the other side. For example, if an artist was to do something crazy like make base or some other type of lower frequency sound move from the left to right or vice versa, you would be missing out on that one channels sound all together. Even if you are running mono and there will be no left to right, you will not here any right at all. Plus having only one rear speaker in the rear playing would just sound strange. I recommend you run both channels into an amp with high level inputs and then bridging it mono in order to get a combined signal of musical information from both channels.