If you are sure about the antenna being the problem, it is very easy to replace since it is external.
You can see it under the bumper. It is a elongated rectangular black box with wires connected to
it. Get the parts from internet, and DIY.
However, are you sure about it?
What "valet buttons" did you mean?
I used a slim screwdriver. I had to mess with the plug, but there is a plastic tab clip that you have to trip before the male/female ends come apart.
Tony,
Try waynemazda.com.
Their part department is very complete and has good prices.