Finding stud bay with plywood over the top plate

Shiseiji

I haven't been able to think of query terms that cover this, so please pardon my ignorance.
30 YO ranch, basic rectangle. ~8" of cellulose insulation. I'm want to drill into a non-structural stud bay to drop a coax cable.

The top plate appears to be covered with 1/2" plywood so I'm scared of missing the center line and drilling into the ceiling. Murphy is my earned middle name. I got two Greenlee extension sets to attack it from the bottom and promptly bound up the 3/4" auger bit so that was a no-go. Had to use a 10" 1/4" box end wrench to give me something to bang it loose. I tried drilling by hand with a ratchet, but the bit bound again.

So as always, TIA for any suggestions.
Dig around the cellulose and measure to drill from top?
Spade bit from bottom?
Cry?

Ron

Shiseiji

Blinding Flash(s) of the Obvious (BFO) :
Plywood is support for flex duct work. Should have listened to myself when I though it was strange they were above the bottom of the wye rafters.
Wall is 2X6 for the duct work and in the middle of the two ducts. Ducts? Wall? We need a hitting self with hammer emote.

Lesson for the day: When the tub caulk job already frustrated you (gee, ops, we cut both these surround corners 3/4" too short, we'll just caulk it up, who'll ever know?), put things away for the weekend.

It'll be funny in two years.

Ron