Per the 69 Mustang Electrical Assembly Manuals, "USE EXISTING DRILL DIMPLE". All 69 radios were an option, so all the accompanying additional items (antenna and ground strap) were mounted on the assembly line or dealer (antenna).
As the 69 Shelbys started out as Mustangs on the assembly line, I recommend you get a set of Mustang Assembly Manuals. You can mark up the manuals as you acquire information. Unfortunately, there are no 69 "Interior" or 69 "Weld & Sealant" manuals but the 70 Mustang manuals are "similar".
Jim