Mirror installation required?

I was recently towed by a tow plane that had no mirror installed. Two way communication was established using VHF and a club member was positioned ahead of the tow plane to relay ground signals. This was a US based club. This launch prompted me to wonder if in the US a mirror is actually required.. After researching 14 CFR and AC 43.13 2B I can not find a regulatory reference to mirror installations in the US. I see various Canadian and European references buy none here in the US. Can anyone point me to a FAR or Advisory Circular that specifies a mirror is required? I just am curious if its a smart practice or an actual requirement for towing. I'm sure this has been covered before but the proof escapes my research.

Thanks
LB