The tekonsha prodigy P3 is a good brake controller (I have that one). There are many threads on installing that in this forum, with pictures to help find the wiring harness which is to the left of the brake pedal, often behind the panel there.
Usually when a hitch is installed, both a 4 prong and a 7 prong are installed. Are you sure the 7 prong isn't there behind a cap? It should be to the left of the hitch.
I believe the Ascent is prewired to support the 7 prong, so if you don't already have one, you should be able to plug it in with the right adapter.
Did you get the OEM subaru hitch? Or a 3rd party hitch?
Pics of your hitch in the back would help too.
Usually when a hitch is installed, both a 4 prong and a 7 prong are installed. Are you sure the 7 prong isn't there behind a cap? It should be to the left of the hitch.
I believe the Ascent is prewired to support the 7 prong, so if you don't already have one, you should be able to plug it in with the right adapter.
Did you get the OEM subaru hitch? Or a 3rd party hitch?
Pics of your hitch in the back would help too.