Rear wiper works on the RAP circuit, and pretty sure it runs down the left side of the vehicle since that is the hinge side.
Most timer relays I have seen will set their internal time when power is lost. They reset when power is applied again. So, a RAP circuit might be good for this application.
But if using an electronic timer relay, I would just recommend tapping into the accessory plug circuit. Since the RAP goes off when the door is opened, that only gives you a max of 20 minutes if you remained in the vehicle.
If you got a timer with 2 hours, then the accessory power would be ok. But the final decision is yours.
Some of the first RAP circuits used by GM were used with timer relays that when ignition power was removed, they would stay powered up for a specified time. Eventually, GM migrated the control of a normal relay via the BCM. The BCM would power up a relay at start up, and turn it off twenty minutes after the power was lost, or a door switch indicated a door was opened.
Now, with the newer BCMs (like on the H3), the BCM operates the RAP circuit directly without any relay in the circuit.
Change is good...
Someday soon, you will see a VCM (Vehicle Control Module) a module that will control all the required electronics on the vehicle; a combination of the BCM, PCM, Brake Control Module, and all other modules. It will be programmable to handle either an auto or a manual, with or without ABS/TC/Handling. It might even incorporate XM and Onstar.