Replace rain sensor
 

Can a backyard mechanic replace the rain sensor without special tools without breaking a costly part? If so how do you do it.

What are you trying to do (tm)?

If you wish to remove the rain/light sensor module from the prism you can release the two slide latches and unplug it.

If you want to remove the purple prism from a windscreen normally the glass that it is bonded to will break.

When mating an RLS sensor to a new windscreen & prism, you need to run an adaptation routine in INPA or DIS.

Rain sensor shows a fault
 

I took it into the dealership and they said they had a fault in the rain sensor so the wipers do not work when using the sensor and the automatic headlights do not work. All I know is they wanted $450 to replace the sensor. Sounds like I cannot do it without special tools. I guess I go without. Thanks for the reply.

Assuming the fault is with the sensor having failed, and not the prism becoming un-bonded from the glass with age, you can do two things:

One unplug the 4 pin connector at the sensor and the wipers will revert to regular pause wipe.

Two release the two locking catches, fit a used sensor (be aware, there are two designs, the LED count is different and they are not compatible) and relearn the sensor to the screen using INPA.