The mirrors you are looking for were specific for 89-91 Suburban and K5 Blazers. They mount just below the the vent wing support post. I had them on my 86 and have another pair of the mounts that use a common mirror housing for Astros and C/K series trucks.
Not my truck but the mirrors are the same. Remember they are specific and hunting them down is tough. They came in power and manual. Good luck!!! Dont put Ford mirrors on a Chevy, Anyone can tell where they came from.