That's an interesting way to wire a relay. Crash makes a great case for NOT using one in this application. Most folks run the fused battery lead to Pin 30. Pins 85/86 are for the lead from the upfitter switch to to actuate the relay switch. 87a isn't an input lead. It's an output and is normally closed when the upfitter switch is off. In other words, when 85/86 aren't on, 87a is hot out. When 85/86 are on, 87a is off and 87 is hot out to your accessory. All you will accomplish by hooking the OEM back up light lead to 87a is have the back up lights on continuously when the upfitter switch is off.
Great, so you can read the instructions that come with a relay on how they are normally used. That is not the only way to wire them. Relays are an electrical circuit that uses a current source to power a coil to flip a switch. They are not directional. Wiring it the way I stated allows the current from the backup lights to flow from pin 87a (used as in input here) to pin 30 (used as the output here) and light the auxiliary reverse lights when the truck is in reverse. When the upfitter switch is on, current across pin 86/85 closes the relay and allows current to flow from pin 87 (used as in input here) to pin 30 (used as the output here) and light the auxiliary reverse lights regardless if the truck is in reverse or not.