Yes, you need to buy the bracket that fits your plug.
Most auto supply stores have a small section and it should have the most common
plug. There also should be a bracket by itself for sale, as they get banged up
often and folks replace them often
Then print out the above pic's for your mechanic, along with the bracket or
leave it to them to buy the bracket and mark up a bit of profit for themselves
I did NOT thread the receiver flange because lazy and didn't want to go back
to the garage for my tape set. So clearance hole in the receiver flange and
double nutted a screw (think 10-32)
Your mechanic should understand all this