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Should be 2 or 3 bolts on the seat bottom facing the rear of the front seats. Remove the bottom seat cushion and then you will see the bolts that hold the back seat cushion on.

If it's a coupe, it's where @Black92GS showed a few posts up. If a sedan, on the back deck as @jiggity76 shows. Regal coupes had a much smaller rear deck due to the more vertical backglass. Top is sedan, bottom is coupe.

That's interesting. My bad...I thought they were all in the same location. My Cutlass coupe is also in the back shelf area. The rear glass is allowing more space for it I'm sure than a Regal coupe.

The receivers are mounted up in the back glass shelf area. The upper rear seatback needs to come out...which means the shelf also needs to come out. Then, to get the shelf out, you have to pull the rear quarter glass interior plastics that go around the glass. GM layered these pieces. Here's my 93 Grand Prix STE sedan. It won't be identical, but it will be the same process and in the same location. That black box up against the rear glass...that's the receiver.

You're welcome.