First off when would a rat have a chance to chew through a cord inside your snakes cage? A live rodent shouldn't ever be left in a cage long enough for that to happen. Live rodents shouldn't be left unattended with a snake for any reason. I've seen too many snakes be killed by unattended rats already.
As for probe placement you have a few options. I personally don't recommend using a single thermostat for more than one heat source, but it can be done. You are going to have to carefully monitor your temps as there won't be anything to prevent disaster on the RHP that isn't being directly monitored by a thermostat.
You want your probe to be placed to that it is centered on the lens of the RHP:
1. You can use hot glue to attach it directly to the lens
2. You can suspend the probe so that it is hanging under the lens
3. You can use hot glue to attach the probe to the bottom of the cage directly under the lens.