Thanks Geoff and Dave, The only image I've been able to find (which is for a GMC) is shown below. At this point I've received enough responses (via this post, via email and in a PM) that I believe (a) either some Chevy trucks did not have this speaker location while others did (b) GMC was different from Chevrolet and (c) coming up with anything that works for everyone probably isn't possible. I'm waiting for a response from a long time friend whose dad owned a 1956 Chevy. It did have a radio and he'll remember but he does work in various ministries so it may be a while before he answers. Thanks again very much for all the help.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
