How do you ensure that PMM sits as a strategic partner to PM, rather than an administrative or subordinate function?

Like any good working relationship, you have to take the time to build trust. Here’s a few things I do to build trust and position myself as a strategic partner to PM.

•  Meet regularly and come with a prepared agenda. It doesn’t necessarily have to be weekly but know how you want to use the time and deliver value to them as well. This might include asking them to walk through the latest roadmap rationale and asking them questions around what challenges they’re running into with testing, adoption and see if there’s an area where you can help them. Whether it's supporting some sales enablement, or connecting them with the product education team, help be a bridge to other teams. 
•  Bring them value - bring insights from marketing that PMs don't typically get access to. Product often doesn’t get deep insight into how your website converts leads, the customer purchase journey, brand perceptions, marketing campaigns - show them how you’re showcasing their product in marketing and what learnings they may take from marketing's latest work. For example, "we know prospects spend 3X time on our pricing page than any other page in our site. Maybe we should consider surfacing pricing on that new product in our app to give customers the information they want" 
•  Pull them into marketing meetings (if express interest in it) where you think their voice is of value. Show them that you understand their goals, what they’re working on and that you value their voice too. PMs want to know the products they're shipping are in good hands, so help them understand the marketing process.
•  If you have a PMM charter / mission - walk them through it. Help them understand what PMM does and doesn’t do. If it’s not well defined at your company or you’re the only PMM, I urge you to do this.