Connecting into just any ol' wire that is switched is not always wise! You may get hold of a wire that isn't fused large enough to handle the additional load you are wanting to connect. It's always best to check the wiring diagram first to see if you really do want to tie into it. The Spyder wiring, from my observation, is not built with very much extra capacity available. If you just connected a relay coil to whatever switched wire you found and fed the real load from the battery through the relay, then that is a proper way to go. Relay coils draw so little current you can add them to just about any wire with no adverse consequence.
Now, in this case the grip heater wiring has the capacity to add the heated seat since that's the way the BRP kit intends. If the RDL seat has a heater somewhat the same wattage as the BRP one, and the owner is OK with the grip heater switches also operating the seat at the same time, then that would be the route to go.
In the case of adding a heated seat with a relay and battery power feed I would suggest you tie into the power in lead of the grip heater. The reason for that is the grip heaters are on the load shedding relay powered circuit so they aren't energized until the engine is running. Avoids adding extra load on the battery during engine start.