If you have a separate bulb for turn signals inside the brake light housing it will be difficult to see the turn bulb flashing when the brake light is on. It would be better to purchase a turn signal switch that uses the brake lights. And probably simpler to do than trying to cob another socket in your housing.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.